Buying help

impossibru

Wait to see them in a daily deal/flash sale. Don't buy anything until the final day if they don't show up in either, just in case it goes on a larger discount later. Happy shopping ;)

Nope

rust on itself will probably not go on sale, or maybe -20% tops
i would say out of the ones you mentioned borderlands 2 is the best and by far (personal preference)

In the best case scenario, all 3 games will go on 75% discount (no dlc's)
5 usd borderlands

5 usd Rust

10 USD saints row 4. No dlcs for borderlands.

And thats be best case scenario.

Rust won't go on a sale: "Sorry - we aren't included in the Steam Sale. We're in early development - we don't want to encourage people to buy right now." (Rust from Twitter)
Borderlands 2 GOTY is 45 $ - quite sure it won't be cheaper than 10-15$
Saints rown 4 possibly 10$, not less. Too new, too big to get 80-90ish, highly unlikely.
It seems you can only buy one of these, maybe if you're lucky (SR4+BL2). Plus for your own financial goodness, I hope you meant only Borderlands 2 GOTY. You'll only need ultimate vault hunter 2 for it - the headhunter DLC-s are not too good - skin + some loot and like a single quest. Not to mention the only skin DLCs, it's money thrown out of the window. (especially considering you don't see yourself in the game at all :D )