If it's really only a test track and you have another 3 lane track already, then why do you need more than one lane? You'll get the most action; in terms of layout, with a one-laner. Have a look at John Warren's (munter) "Anaconda" track; it's a nice one-laner.
On a test track I would go for as many track variables as I could get: a medium-sized straight to top end, a sweeper, as already mentioned, for track holding power (a big sweeper is almost like a skid pad in that respect), a chicane, an increasing radius turn, a decreasing radium turn, a hairpin, a hill, a drop, etc. All the elements that your car is apt to encounter in a racing situation. A one laner will give you the most track space in your 17'x6' area.
Now, if the 3 laner is only raceable during certain times of the year and you want to be able to race all year round, then that's a reason to go with a 2 laner.
Anyway, have fun with it!
Allan