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Summary
Playing in a game with a significantly higher level player is bad for both parties. Lowbies get penalized by receiving less experience per kill just by having a high level character in the game. However, they will also gradually catch up to a higher level characters because the higher level character is getting almost no experience killing the low level monsters. Although there is a lot of conflicting information floating around, this seems to be supported by a recent Q&A with Blizzard.
Guest-224: Say for example that I am playing a high level character, but I want to play with my friend who only has a low level character. Will me joining his game ruin the experience for him because I could just blast through the content and make it too easy for him?
JayWilson: To Guest-224 You could blast him through the content, but his XP will be greatly diminished to the point that it's not very beneficial for your low level friend...unless he likes spoilers. But that's what Youtube is for.
For more details, read on.
Twinking is penalized
While it is true that there is no party experience split, there also seems to be an additional game mechanic to penalize twinking.
I captured some measurements when playing with several other noble Gaming.SE heroes, and I found an interesting result. For reference, in the case of interest, my level 12 character was playing with a level 3/level 4 character. Clearly, there is no party experience split, because when I was in range for experience sharing, and when I was out of range (say in town), party members still got the same amount of experience. So the fact that I was sharing experience did not affect how much experience they received.
However, I wanted to be double sure that I wasn't influencing anything, so I left the game altogether. When I left the game - my former party member got demonstrably more experience per kill, on the exact same monsters. Doing some back calculations, I find that the party member at level 3/4 seemed to get ~60% less experience when I was present in the game with my level 12 character. I couldn't figure out the exact calculation, but I expect it also varies based on the level difference. The level delta seemed to have no effect on the experience I received, but I was already getting very little experience due to the low level of the monsters. For those interested in the numbers, I recorded multiple results in two scenarios.
Level 3 with Level 12: 11xp per kill with level 12 present, 28xp per kill with level 12 absent
= ~ 61% less
Level 4 with Level 12: 18xp per kill with level 12 present, 41xp per kill with level 12 absent
= ~ 56% less
But there is more to consider...
In early levels, it will be hard to compare experience differentials based on level (as is done in this question), because the absolute amount of experience required to get to a level is so small.
From this excellent Google Doc that RavenDreamer linked in another answer, you can see the experience needed for each level. If we consider your example where one character had a head start at level 3, the may have had as little as a 2700 xp lead. To get to level 6 you would each cumulatively need 9k, and to get to level 7 you would need 11.7k. So if each character got ~9k, with the initially level 3 char getting ever so slightly less (100 xp for example), then all characters could end up level 6 together.
When you also consider experience bonuses like the Massacre bonus, and the fact that if members of your party strayed at any point during the session then not all experience may have been perfectly shared due to range, plus the fact that monster experience depends on your level... It seems clear that if you slum it with low level buddies, when you yourself are not at a huge advantage in terms of level, then your buddies are likely to catch up.
