I want to make a R/D trilam. Target is 66"-68", 50#@28. I have hickory,cherry, maple and ERC to choose from. For core iwas thinking maybe ERC or cherry, I haven't worked with them myself but I've heard they're finnicky for bellies and I don't really wanna deal with that. So maybe Maple/cherry/Maple or Hickory/cherry/maple? What do you guys think? Also for the core, can I use 1/4, rift and plain sawed for that and what about runoff?
Thanks!

Maple/cherry/maple. Also heat treat your belly lam. Maple is wonderful in compression after it's been heat treated. Grain orientation in the core doesn't matter.

Grain orientation doesn't matter, but try to keep runouts to a minimum.

Maple/cherry/maple or maple/maple/maple

Great! Pretty sure I'll do it M/C/M. I was thinking 1 3/4 at the fades out to a third of the limb then taper to 1/2", does it seem reasonable?

Very reasonable, you could even go full width out to half the limb or more without much downside, but regardless I think 1.75" is good for width. You could probably go narrower, but I personally like to play it safe.