You have 2 leopard geckos and a desert iguana is it safe to keep the 3 together in a 29 g long tank?

No! not under any circumstances is that ok. First of all, logically consider this: Leopard Geckos are mainly from Pakistani deserts, and the Desert Iguana is from the American Southwest. Leopard geckos may be carriers of Middle Eastern diseases that they have adapted an immunity to, while the iguana may be susceptible, and vice-versa. This is just one reason why two species of different localities should never be put together, not to mention aggression issues. Plus, if you're keeping your iguana remotely decently, it should have AT LEAST a 40 gallon tank, a high powered (125 watts or more) Mercury vapor bulb that emits UVB (not just UVA) like a Powersun. Leopard Geckos may be harmed by this level of UVB and should therefore have a separate tank from each other unless they are two females of the same size.