Rinoa's card:

Get Ifrit's, lose it to her father in Deling City, Play him again for Rinoa's. Regain Ifrit's from the headmaster of galbadia garden in F-H

Squall's: Get it from Laguna in disc3 on the Ragnarok

Irvine: Give the card queen something or other, and get it from Flo in F-H

Alternatively, you could get any cards you missed from the Queen of Cards in Disc 4 in the crash site outside of Esthar.


Oh, and the point of a low-levels game isn't the difficulty.
It's either so you can abuse the abilityx4+str/vit/mag/spr+ abilities in discs 3 and 4 or to make the entire game easier.