There sure is – one way is using the good ol' conditional operator:

  (x==3)?sin(i+t):0

Result: https://doersino.github.io/tixyz/?code=%28x%3D%3D3%29%3Fsin%...

Another is multiplying with a boolean expression that's automatically cast to 0 or 1 depending on its value:

  sin(i+t)*(x==3)

Result: https://doersino.github.io/tixyz/?code=sin%28i%2Bt%29*%28x%3...