I am going to send a wedding gift 3 months late and not mention it until two months after the wedding.
Why would you do that. Can you not afford a gift right now?
I want to give them a surprise.
I understand that you want it to be a surprise, but it's expected that people will bring gifts to a wedding if they can afford one. Showing up to the wedding without a gift and without an explanation will confuse the bride and groom. You should at least tell them beforehand that they will be getting a gift from you  in a couple months.
I don't care what you're saying and I will do what I want to do.
Ultimately the choice is yours, but it's generally expected for gifts to be brought to the wedding and while you want to surprise the couple they might actually think of the gesture as rude. If you want to surprise them, why not try to think of a really creative gift that they won't be expecting?