at level 5 with a +3 spell casting modifier. Does this max at 3d8 or can each d8 explode up to 3d8 making the max at level 5 with a +3 6d8? 

I read it as the first, but it very well could be the later.

This makes up for the changes to Chill Touch. Looking forward to using this with a quickened leveled spell.

at level 5 with a +3 spell casting modifier. Does this max at 3d8 or can each d8 explode up to 3d8 making the max at level 5 with a +3 6d8? 

I read it as the first, but it very well could be the later.

 It would max at 3d8. "the maximum number of these d8s you can add to the spell’s damage equals your spellcasting ability modifier." 

It specifies the maximum number of d8s that can be added to the damage, not the maximum number per die.

Wouldn't the max be 5d8? Because 2d8 is the standard and you can ADD dice up to your modifier to that damage so 2d8 base and up to 3d8 added to that if you roll great. Makes 5d8 total in my opinion.

This is how I would interpret it as well. Let's go 40 damage cantrip.

I would agree that the max would be 5d8 at level 5 with a +3 modifier. 

You get to add a d8 for any 8 rolled, including any of the added d8, up to your modifier. 

Hypothetically, a level 1 sorcerer with 20 (+5) charisma could roll an 8 on their 1d8, then add another d8, roll an 8, add another d8... up to 5 times for 6d8 total. Giving a 1 in 16777216 chance of doing 48 damage 

I'm pretty sure it is just a typo, but the language for this spell requires you to "Make a ranged attack roll" and doesn't specify "Ranged spell attack roll" as other spells do.   Could you use Dex for this?

This is a really good cantrip, and I am guessing it is the successor to Chaos Bolt (due to both being Sorcerer-exclusive spells) since Chromatic Orb got it's bouncing mechanic. This thing can do some serious damage, especally as it increases in damage and/or you crit, as that means more chances for more D8s!