Triangle Area Side Side Side

APL 21 20

Takes screen input via ←⎕

(×/(+/t÷2)-0,t←⎕)*.5

J, 23 19 chars

   (4%~2%:[:*/+/-0,+:)

   (4%~2%:[:*/+/-0,+:) 2 3 4
2.90474

   (4%~2%:[:*/+/-0,+:) 3,4,5
6

17-char version if input is in i: 4%~%:*/(+/,+/-+:)i

original 23-char version: (%:@(+/**/@(+/-+:))%4:)

Mathematica 23

√Times@@(+##/2-{0,##})&