The following direct upgrade paths to UCCx 8.0(2) is listed in the "Cisco Unified Contact Center Express (Cisco Unified CCX) Software and Hardware Compatibility Guide": 5.0(2) SR2, 7.0(1) SR5 and 8.0(1).
Can UCCx 5.0(2) SR3 be upgraded to 8.0(2) directly?
Reading the compatibility guide it does state it was updated on the 4th June 2010 to include 5.0(2)SR3, but it is strange it's not included in the direct upgrade paths (in fact SR3 is not mentioned anywhere else in the document).
Given that SR3 was merely a bug fix release, I would imagine it should be fine anyway. I have done other "undocumented" upgrades before with UCCX & CUCM without problem. But there is no gaurentee so take the usual precautions. Or even consider regressing to SR2 first if you can't take the risk.
I have a similar upgrade to do at some point, hence my interest, but it's not for some time yet.