In short, the answer to your question is…no.
Because 3rd gear is a lower gear, the engine is turning over faster, thus meaning that there is more power instantly available. If he was in “top gear”, and if he floored the gas, the transmission would go into 3rd gear at that point in order to accelerate.
That being said, what he is doing really makes very little sense, because the engine is consuming more gas, possibly more oil, and is wearing at a faster rate as a result of his refusal to put the car into its normal cruising gear.
And, if he is doing this in order to avoid braking, that implies that he is not allowing sufficient following distance behind other cars. When I am on the highway, I rarely have to brake, simply because I leave enough following distance so that simply backing off on the gas pedal slows me down enough if somebody in front of me is driving too slowly.
Truthfully, he sounds like somebody who–in addition to being nervous behind the wheel–also has little or no orientation to what he should be doing when he drives.
My suggestion–which he will probably reject–is that he first take driving lessons.
Yes, the same lessons that a beginner/learner takes, simply because I think that he still has a lot to learn.
Then, once he begins to drive with more confidence, and once he ceases doing nonsensical things like driving at high speed in 3rd gear (and likely tailgating other cars), it might be time to consider a new car. However, it is important that he actually learns how to drive properly before you get that new car.