As for driving with only three studs - it'll work, just don't Autocross
with it! ;-).
For installing the lugs, you need to understand that the lug holds due to
the knurl pattern on the replacement lug. If you could just tap them in,
they would not "hold" very well when you torque them down. In machinist
lingo, the hole for the stud is reamed to an interference fit for the
"shank" of the stud. The knurls actually stand proud of the shank and are
pressed in to the hub with a substantial amount of force.
By way of example, I recently installed some bigger studs on my TR6 and we
needed to use around 3000 pounds of force to get the studs seated. Granted
the studs I'm installing are considerably bigger than the stock ones, but
the bottom line is: you don't tap them in.
One thing to keep in mind when you follow the prescribed method: the shank
is a bit wider than the threaded portion of the stud. Make sure that your
washers allow for that little extra diameter of the stud when you tighten
down. The reason is that the shank will stand proud of the hub by a little
bit, and if your washers are too small, the stud will bottom on the washer
before it is fully seated. You'll notice this the first time you mount the
wheels to the recommended torque and 5 miles later all the lugs are loose!
the other thing with the rears is that the flange for the hub that bolts to
the trailing arm really interferes with getting a clean blow to the head of
the stud. And if you miss, you'll wreck your hub... guaranteed.
And make sure that you pull straight when using the washer method. If you
do not, you can bend the hub flange. I've been down that street, and it
TR6 - with mondo studs!