this is a complete donk here .. he's never overbet the pot like this though .. he prolly doesnt think i have an A since i checked but i dont think i can call this or anything, whats the right play here? im only invested the BB so i think i should just fold here....

PokerStars No-Limit Hold'em, $0.25 BB (6 handed) - Poker-Stars Converter Tool from FlopTurnRiver.com

SB ($25)
Hero (BB) ($32.35)
UTG ($32.65)
MP ($24.25)
CO ($12.35)
Button ($28)

Preflop: Hero is BB with A, 5
2 folds, CO calls $0.25, 2 folds, Hero checks

Flop: ($0.60) A, 7, 3 (2 players)
Hero checks, CO bets $3, Hero ???