For HTTP, you rely on nobody being able to spy on your packets (or alter/drop/duplicate them) between your device (your smartphone or computer) and the target server. 2G and 3G offer some encryption, but only from your device to the nearest base station; encryption is "over the air". From the base station to the target server, there is no encryption (or, at least, nothing is standard-enforced), so you cannot assume that the data is protected. Besides, the 2G encryption is based on A5/1, which is known to be weak (and there are ways to force a given cell phone to switch back to A5/2, which is weak by design, and can be cracked in real-time). 3G uses KASUMI which, while academically weakened, is still beyond cracking with today's technology. However, I repeat, 2G/3G encryption is only against nearby adversaries, who limit themselves to the radio link between your device and the station. Don't use HTTP for transfers of valuable data.
HTTPS is HTTP-within-SSL. SSL has been designed to offer security (server authentication, confidentiality and data integrity) regardless of the transport medium. 2G/3G is irrelevant to the security of HTTPS. It depends on your borwser / operating system (and, in particular, which certification authorities is accepts as "trusted roots"), not on the network.