If, as you say

Which one would you use, and why?

If, as you say, Martin is coming here tomorrow, we can...

If, as you say, Martin will come here tomorrow, we can...

If, as you say, Martin is going to come here tomorrow, we can...

If this’s a Type I conditional (i.e., it’s quite probable Martin will show up tomorrow), you could use the simple present in your if-clause. I would write: “If, as you say, Martin comes here tomorrow, we can...”

If this’s a Type II conditional (i.e., there’s remote chance Martin will show up tomorrow), you could use the simple past in your if-clause. I would write: “If, as you say, Martin came here tomorrow, we could...”

AVOID using future in the if-clause of conditionals.