AFAIK there's only one that claims to meet DOT standards for street use. Shroth (sp?). I called about it when they first came out. At least at that time, they would not fit the standard Mopar and GM seatbelt bolts of the 60s and 70s. (Nor do most 5 and 6 point harness - the holes have to be opened up).
Do your homework. The plus side is obvious. They can be used with a regular car seat. The neg side is harder to nail down. I'll mention a couple potentials I'm aware of.
* Last year I read a regulation were 4 points were prohibited, but I can't recall if this was NJMP's interpretation or NJ State racing regs themselves.
See what's allowed where you intend to play (and drive).
* Sternum damage in a forward accident. A mid chest cross strap would might reduce that but its probably been tested and modelled. Worth finding out.
* Possibility the shoulder straps pull the center of the seat belt up as the torso moves forward. This can allow the hips an dbody to slip under the lap belt ('submarining'). I assume Schroth figured out an attachment location etc to prevent this, but again worth checking into.

If you go with a racing seat, there will be a slot for anti-submarine belts. If you go with a more supportive automotive seat, maybe you'll be happier with the 3 point than you are now.