I am not sure about IF

We need following

A Filed--- THIS

B filed --- IS or Blank

D field is like A+B+ C to show

but be fore each filed add "$"

if B filed is "is"

D filed show

$THIS$is$it

like this .

but if B filed is Blank .

i can do only

$this$$it

But we need to show 

$this$it

how do i set up in Formula .

if can not how do i do in function,