It depends on the density. The density of a planet can vary widely depending on its composition. Looking at my worldbuilding spreadsheet:
A pure silicate world with no significant metals and a mass of 7 M(Earth) would have a radius of 1.81, a density of 1.18, and a surface gravity of 2.14, all relative to Earth. A 7-Earth-mass world with an Earthlike composition (an iron core making up about 32.5% of its mass) would have a radius of 1.68, a density of 1.48, and a gravity of 2.48. A 7-Earth-mass world with a Mercury-like composition (iron core making up 70% of its mass) would have a radius of 1.49, a density of 2.12, and a gravity of 3.15. These are idealized cases, especially the first one, but they should give you ideas of the range of possibilities.
Or let's say you make it an ice planet with the composition of some of Saturn's moons, about 50% ice and 50% rock and metal. In that case, at 7 Earth masses, the radius would be about 2.33 of Earth's, the density only about 56% of Earth's, and the gravity only 1.29 gees. (Well, actually what I have in the spreadsheet is about 53% rock and metal and 47% ice by mass, but you get the idea.)
As for the ability to perform in high gravity, it would be extremely difficult at first, but if you trained properly and avoided injury, you could get strong enough to cope with it, just like any other kind of physical training. However, if you did strain or injure yourself early on, it might be hard to heal properly and the effects could be crippling. Also, in the long term it would put more strain on the circulatory system, but again, I think that's a question of training. I've read stories postulating that people on high-gee worlds had more heart failures and shorter lives, but I'm not so sure that's accurate. The heart is a muscle, so the more use it gets, the stronger it gets (again, assuming excessive strain and injury are avoided). So I'd think that, as with the rest of your muscles, it's a question of whether you train it properly, whether you get to start out slow and build up your strength and endurance. However, if you're suddenly dropped into a prison planet with 2-3 times your accustomed gravity and have to start fighting or struggling right away, that's more likely to lead to overstrain and injury and make it difficult to function. Although if you're already very physically fit to begin with, your chances would be better. (Needless to say, your cardiovascular fitness could be negatively impacted by factors in the prison environment such as stress or tobacco and drug use. Clean living is your best bet, but if you lived clean, why would be on a prison planet?)
Of course, all this is assuming normal, unaugmented humans. If you bring genetic and technological enhancements into the mix, all bets are off. People sent to live on high-gee worlds could have their joints reinforced or replaced with superior artificial versions, could possibly have their muscles augmented by nanofiber cables that contract with much greater force than human muscle, could have their heart and organs similarly augmented, etc. Or they could just wear the kind of strength-enhancing armature that's already being developed for military and medical uses.
