So I am gonna be playing for the first time soon and when making a character my DM let me know the rules for how he wants us to roll our stats.

We roll 4 D6s and reroll any 1s or 2s. The lowest one gets nicked and we get the combined number of the last 3. If we roll 3 of the same number regardless of the number it automatically turns out to be a 19.

I feel like it will be OP really early as my fighter has a 20 in strength at lvl 1 and int is his lowest at 14. Is this normal for new players or does my DM just want us to be op?

It’s not a standard rule, but it’s not super unheard of for a DM to tweak the roll for stats process or tell the table to take max HP rather than rolling for it or taking half.

I can't tell what your DM want, but all i can tell is that it's not the norm

That's 90% correct. You don't normally reroll any dice regardless of their score, but rerolling low dice is a common house rule. The getting 19 if you get three of the same us also not part of the rules, and I've never heard of it before even as a houserule.

That would produce a very powerful character. However, so long as everyone has the same rules, it's fair, and it's up to the DM to make the encounters fun, so it's his problem. Personally, I'd ditch the get 19 if you get three numbers same rule. It's likely to increase disparity in the group and make it less fun.

4d6, drop lowest is the standard. Everything else you've listed is not. Using those rules, player characters will end up being way, WAY more powerful than normal.

Maybe he just really wants to use less common adversaries for lower levels and make sure it won't be a slaughter. Seems like he'll be able to seriously up-tune the encounters you guys will be facing from the get-go. 

It’s not incredibly uncommon for a DM to adjust how stats get rolled. I will say that starting with a 20 in your main stat is unfortunate, and if it were me I would switch my stats around to have a lower number in strength (like a 16ish) and stick that 20 in Constitution or something. Not because you’ll be “OP” or anything, but because you won’t get an opportunity to feel your attack bonus increase as you level up by putting your ASIs into your prime stat. That’s a satisfying thing that you’ll miss out on, the only way you’ll get to improve your attack modifier is when your Proficiency bonus goes up. I’ve had a character start with an 18 in their prime stat, so I speak from experience.

Quote from Qazplmwer >>

So I am gonna be playing for the first time soon and when making a character my DM let me know the rules for how he wants us to roll our stats.

We roll 4 D6s and reroll any 1s or 2s. The lowest one gets nicked and we get the combined number of the last 3.

I recall that part being a common rule among some games in the mid 80's.  That fell out of favor for the 4d6 rule way back when, but some folks still stuck with it.

If we roll 3 of the same number regardless of the number it automatically turns out to be a 19.

I feel like it will be OP really early as my fighter has a 20 in strength at lvl 1 and int is his lowest at 14. Is this normal for new players or does my DM just want us to be op?

That part is a shock to me, personally -- but I confess I am a point buy person after a long argument I lost in the 80's, lol.

It does and would create characters who scores are significant, and statistically well above the median, so pretty obvious the PCs are meant to be hardcore "best of the best". 

If I were to do that I would expect to to run a conan style campaign.

I don't see it being a problem if you're DM is prepared to deal with characters with higher than average starting stats. 

You'll only be OP relative to other characters (not ones in your game) that use a more traditional method of determining starting stats if your DM runs encounters (combat and non-combat) commensurate to your party's abilities. 

Worst case, it's "trial and error" for your DM. He has enough ways to tweak skill checks and combat to still make it fun.

Or just is tired of characters with dump stats, or wants his players to feel like they have “real” hero’s - especially if most are first time players. I have never been a fan of point buy etc over rolled stats. Instead of reroll Inc 1s & 2s  I told the players in my campaign to roll and take the first set that had nothing below a 10 and at least 3 rolls of 13+. No dump stat, and good enough to do multiclasses if they wanted. If they got lucky they had really good stats but then “hero’s” often do start off ahead of the game.

Quote from Wi1dBi11 >>

Or just is tired of characters with dump stats, or wants his players to feel like they have “real” hero’s - especially if most are first time players. I have never been a fan of point buy etc over rolled stats. Instead of reroll Inc 1s & 2s  I told the players in my campaign to roll and take the first set that had nothing below a 10 and at least 3 rolls of 13+. No dump stat, and good enough to do multiclasses if they wanted. If they got lucky they had really good stats but then “hero’s” often do start off ahead of the game.

Quite true on the possible motivation, ime.  Been there, lol.

I offer a roll, point buy, and assigned score set of options because it lets folks work the way they want. Long ago and far away, I lost an argument, and let it ride. I tend to do the point buy because it lets me keep them at a rough given mean, and lets me do things like this DM is probably trying to do -- get away from the 9 to 13 scores that do so often include a dump score.

4d6 drop the lowest and reroll ones is the norm for my group I can understand expanding to include rerolling twos the only reason i can see the reason to increase to 19 would be that he considers you to be “The Heroes” ™.  While it is not the norm from most of our perspectives it is well within the scope of DM to change how characters are created.  When i first started back in the 1980 my DM offered 3d6, 2d10 or 1d20 per stat.  While it may seem overpowered now   it is a boon to you and trust me it is exceptionally easy, ridiculously easy to put the players into a scenario where their massive stats, special abilities, and spell power are of no use. 

Sounds like a brand new DM.

I suspect sometime before making level 3 everyone will have magic weapons and protections. 

The first part of how you describe rolling is fairly common and make sense. But the 19 is just crazy talk.