I am sorry 2 hear so many people are having so many issues with MMS / Juicy's R3 rom.
These are the steps I took right after the Juicy R2 SPB Backup and flash to R3. Hopefully this helps out someone with a similar set up.
Pre Flash: Set up XDA_UC w/ all necessary cabs. (Note: I am using
OrigDefaultWMtexting4Manila.cab to keep my original screens)
Post Flash: Choose Sprint Carrier.> Device Restarts > Clear Storage > Start > Click Customize > After it loads all CABS, Scroll to Messaging Tab > All Messages > Press menu, scroll down to Pics/Video settings > Go to cdma servers tab open the existing server > Name the server whatever you like. (Sprint) > leave gateway blank >port number is 80 > server address:
http://mms.sprintpcs.com/servlets/mms > connect via: Sprint > Press done.
Tested from one Sprint phone to another. incoming & outgoing (speed varies)