Make a Surfin' Word

Dyalog APL, 37 bytes

{⍉↑v⌽¨⍵↑¨⍨v←(-∘1-⌈/)⌊.5+⎕×1○⎕-⍨⎕×⍳≢⍵}

Takes text as right argument, and then prompts for m, b, and a.