Is not hearing back in a few days mean they are not interested?

You can't read anything into 3 days. If you're serious about your proposal, I'd expect between 3-5 days after sending it, you'd give them a "curiosity call" to "make sure they received your email" and of course to make sure they didn't have any further questions. That phone call will tell them that you're serious about the proposal and will allow you to feel out for their initial vibe. If you're looking for funding for a project though, I'd say you'll be waiting around a fortnight to a month before you get a definite confirmed answer. In that time you should have spoken with them both on the phone and through email correspondence a number of times.