From at least FF8 they stopped using numbers for spell levels and started adding suffixes. The second level would be -a, so for example Fire would become Fira, and the third level would add -ga onto the suffix so Fira would become Firaga. I suppose it's good they never went higher than three levels, because it would sound pretty damned silly.
"Firaga didn't work! We need something stronger! I have to put everything I have into this... FIRAGAGA!"
"You're not alone! THUNDAGAGA!" "BLIZZAGAGA!" "WINDAGAGA!" "RADIOGAGA!"
"...what? Oh, right, that's a song not a spell, I get them confused sometimes. Just be glad I didn't use the ultimate slime summon spell."
"Let me guess... Goo-gaga?"
"Nobody's used that spell in three hundred years... it's too damned embarrassing to call it out."