We use the Multi-Tech MultiVOIP line of SIP gateways with the 3300, had very good luck with them, although I will be honest and say we have never used them for faxing. On our hosted side of the house (Broadsoft SIP server), they use Audiocodes MP-112/114/118 gateways with really good success! We tried using the Grandstream HT-502 for fax applications but had hit and miss luck with them, if I was going to fax I would definitely go Audiocodes.
Attached is Mitel's latest SIP interop guide, you will notice few SIP Gateways that have been certified to work for faxing (T.38) with the 3300, Audiocodes being one of them. Hope it helps!