That's a rough guess. You don't want the pump to run dry. I usually have at least a 4 inch drainage or false bottom in all but the smaller (10-15 gallon) tanks. 3" doesn't seem too much for a 125 to me. You'll still have a lot of "head room" for substrate and plants.
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Patty
Pahsimeroi, Idaho

Dendrobates: auratus blue, auratus Ancon Hill, tinctorius azureus, leucomelas. Phyllobates: vittatus, terribilis, lugubris. Epipedobates: anthonyi tricolor pasaje. Ranitomeya fantastica, imitator, reticulata. Adelphobates castaneoticus, galactonotus. Oophagia pumilio Bastimentos. (updated systematic nomenclature)