well, off the top of my head, I'd say :
* Hit points (it takes a lot longer for 3 vs 5 PCs to take down a sack of HP; maybe something like down 10% - i might suggest going a tiny bit lower than that, like 15%-20% down at most, if the missing PCs are the heavy hitters of the group or else you'll get stuck in a grindy combat)
* Damage (damage can't get spread around to 5 PCs and instead concentrated on the 3, making it deadlier and fewer allies to get around to helping the PCs if one goes down. Maybe something like -2 damage or -4 damage per attack might suffice - I'd probably lean towards the -4 damage if the missing party members are the leaders/defender types of the group)
* Skill Challenge complexity. If there is a skill challenge that acts as an Achilles heel to the solo (ie stop the ritual he's guarding, or shatter the gem that feeds his powers and weakens him, etc) then lower the number of successes needed since there are fewer PCs available to contribute to the skill challenge and keep the solo occupied at the same time.
Stuff that shouldn't need to get changed:
defenses, attack bonuses, initiative, etc.
Something that is tricky:
tactical spacing... the PCs will have a harder time flanking or holding a line or getting the solo busy with some while the others hang back, etc. you can't really account for this in a clean and easy manner though, so (short of adding in companions or random allies that might be there to help) ... i think it's one of those things you'd have to accept as a fact of missing players and not adjust...
(I'm probably missing something obvious. but I'm tired so this is all i could think of at the moment

-- but hope that helps.)