A tip from someone that gets really overwhelmed with choosing discs: I might start with Latitude 64 since they've got the biggest catalogue and try to make a bag with only their discs. Then supplement with the other two brands. It shouldn't be too hard to find something you like since the whole Trilogy brand has a ton of discs to choose from, but giving yourself a restriction might be a good thing.
Or, alternatively, you could just look at the brands' most popular discs and just go with them. Tried and true.
The first suggestion, for my bag personally, I might do something like:
Ballista Pro/Ballista
Grace
Felon/Explorer/River
Anchor/Maces (I'm a Roc thrower, so I like the beaded "old-school" feeling mids)
Harp/Emac Judge
The second suggestion, your bag might look something like (I'm not up to date on all of the most popular molds, but these are ones I see thrown a lot):
Enforcer/Ballista Pro/Grace
Felon/Saint Pro/Maverick
Verdict/Emac Truth/Fuse
Harp/Emac Judge/Pure