I am a developer and get offers like that on a weekly basis. I do not consider most of them. Reason: I have many ideas of my own I want to develop. You can have ideas on every corner and in all of the cases one cannot say if one is working or not.
Now, why should a developer work for free?
You need to give good reasons for that, because if one works for free, he sacrifices family time or time he would spent for his own projects. Or hitchhiking time. Or what else.
If you want to attract a developer, you should give more than 3% equity. He should have at least as much as the other founders.
This is a post on the "idea people" which has spread among the developer community:
If you would try to attract me, I would like to know what you can bring into the startup. "I have connections" is not good enough, as you are not impressed when I would say "I have a computer". What would you do once the startup is running? How can you contribute in future, once the "idea phase" is over?
I once developed an app with an "idea guy" (everybody makes mistakes). The app was ready, but my partner was not able to do anything else than give me his ideas. But I would have needed support in marketing, speaking with the target group and so on. This is what he should have been done as he was actually part of our target group. I had to learn that the idea is worth nothing if you don't have partners.
That all being said:
With just what you wrote, you expect too much.
You need to put some more on the table: how would the future look like when there is funding?
Besides, I would not agree to your offer if there is not a reasonable equity.