316L will work just fine. You're dealing with IP here if you're on the low side of the regulator - which is only around 135 PSI (give or take a little.) Wall thickness will depend on several factors. Primarily, given that you've spec'd material, how big a hole are you drilling on the inside to line up your ports? Once you decide how big you want for gas flow, it's pretty straightforward to run the calculations for the pressure vessel and decide your wall thickness...but you're currently one-degree-of-freedom underspecified, at least with the assumptions of intermediate pressure and 316L that I've made.
While I'm all about DIY, have you considered these two:
Silent Diving Direct or
Silent Diving Direct? The larger (and strangely, cheaper) one is two split halves in a single block, as the oxygen and diluent both split out in the manifold.
Alternatively, I've used this piece -
Golem Gear, Inc. - Y Block - 3 ports before and found it to more than meet my needs. Golem Gear / Jakub is a great guy to deal with and they have other similar manifolds which may fit your needs.
As much as I will encourage you to build this, the cost of buying one pre-made for the appropriate threads is just so inexpensive that it seems like more trouble than it's worth when you take into account having to do a couple set ups on the lathe and mill, the annoyance of tapping and deburring the holes, finishing it, degreasing / O2 cleaning it, etc. I know my time is worth far more than the couple of hours it'd take me to whip this out in the shop, and while I enjoy it tremendously, there's more complicated things to build that are probably a better use of my / your time.