Doing the math.
As I see it, the answer is no.
I think what you meant to ask was, "wouldn't 3 characters take nearly twice as many rounds to do the same job?" in which case the answer is (almost) yes.
Suppose that said solo takes 10 rounds to kill. For 5 characters, that's 50 man-rounds. And the monster gets 10 rounds as well, for a total of 60 man-rounds. Assuming 90 minutes, that's 1.5 minutes for each character's action. Probably not realistic, but it makes the numbers easy.
Now, assuming just 3 characters, they should only require about the same 50 man-rounds, assuming their average-damage-per-character remains similar to the first group. That means (almost) 17 rounds per character, a 70% increase (not quite "twice as many rounds" but close enough for government math). But now the monster gets 17 rounds instead of 10, for a total of 68 man-rounds. Assuming the same 1.5 minutes per round, we have gone from 90 minutes to 102 minutes.
Not a huge difference between 90 minutes and 102. About a 13% increase - far from "twice as long".
True, except for one thing. All man-rounds are not created equal. PCs get a limited number of "power rounds," when they use their encounter and daily powers.
We're talking 10th level here, so I think it's reasonable to figure each PC should have about five "power rounds." Say a "power round" is on average twice as effective as a normal round, which I find to be generally the case. And we'll also figure that one round of attacks for the monster takes twice as long as one round of attacks for a PC, since solo monsters usually have a lot of special powers.
If it takes 10 rounds for a team of 5 PCs to kill a solo, that means the PCs have to inflict a total of 75 rounds' worth of damage (5 power rounds plus 5 normal rounds for each PC). So, you're looking at:
5 PCs versus one solo: Each PC takes five power rounds and five regular rounds, for a total of 50. The solo takes ten rounds, each one counting double, for a total of 20. The combat takes a grand total of 70 rounds.
3 PCs versus one solo: Each PC takes five power rounds and
fifteen regular rounds, for a total of 60. The solo takes twenty rounds, each one counting double, for a total of 40. The combat takes a grand total of 100 rounds - an increase of nearly 50%.
It still isn't double, but it's significant. Moreover, it's much more likely to get boring, since the PCs will run out of encounter and daily powers and then have a lot of hit points still to chip away with at-wills.
I
have noticed that when I've played with only three players, combat feels draggier than when we've got four or five. Possibly that's the problem.