Hi

I understand that if a player starts a turn with 0HP they have to make a DST. Do they just make one DST per turn, or do they continue rolling until they achieve 3 successes/failures?

Thank you

Paul

Whenever you start your turn with 0 hit points, you must make a special saving throw, called a death saving throw, to determine whether you creep closer to death or hang onto life.

Basic Rules, Death Saving Throws.

https://www.dndbeyond.com/sources/basic-rules

It's the start of every turn.

At the start of every turn until they either have 3 successes, at which point they remain at 0 HP but have stabilized, or they have 3 failures (a nat 1 counts as 2 failures), at which point they are dead instead of unconscious. Exception: rolling a 20 brings them back to 1 HP, so they now are conscious. The 3 successes or failures don't have to be consecutive, it's just total.

A stable creature doesn't make death saving throws, even though it has 0 hit points, but it does remain unconscious. The creature stops being stable, and must start making death saving throws again, if it takes any damage. A stable creature that isn't healed regains 1 hit point after 1d4 hours.

Getting stabilized also resets the count.

Here's the full rules on death saving throws.

Once per turn. 

If you have something that increases "saves"  or  "saving throws"... will it work on death saves too???

Death Saving Throws are Saving Throws.

PHB p.197 

You are in the hands of fate now, aided only by spells and features that improve your chances of succeeding on a saving throw.

Then if you get a 22 it just counts as a 20???

And if roll a 1 with a +1 totalling a 2 it doesn't not count for a double failure???

If you get a 20 on the die, no matter your bonus, you regain 1hp.

If you get a 1 on the die, no matter your bonus, your take 2 failures.

If you get a 9 on the die and have at least a +1 bonus, this is a success.