It would be the bare mnimum for the types of frogs you are considering. (Think 10 gallons per frog). The 12x12x18 will yield about 10 gallons of space after your substrate is in. Just do your research and plant / set it up with plenty of hides. A 12x12x18 is an affordable means of getting gointo the hobby.
A 12 x 12 x 18 Exo has interior dimensions of about 0.5" less - so 11.5 x 11.5 x 17.5. Two inches under egg crate/ false bottom/Matala plus 3" ABG = height of 12.5. This gives 11.5 x 11.5 x 12.5 = 1653 cubic in = 7.2 gallons.
On "affordability": An 18 x 18 x 24 has 2.25X the footprint, 3X the volume, and is only twice the price. I know that "affordable" usually means "cheapest", but maybe another metric is more appropriate.
Not knockdown arguments here, and I'm not giving any recommendations, but these are things to think about. I like to think of 'bare minimum' situations as being reasonably recommended only to very experienced keepers, since they have the knowledge needed to successfully live on the edge. Those with less experience should use sufficient equipment to buffer their lack of knowledge and experience.