Modification of vehicle table?
 

My group has a Soviet character, and that player would like to have a BRDM-3. I am unsure if this would be okay for the group to have in the Kalisz starter adventure, or if I should tell them that they can only generate from the given table in the boxed set.

Unless it's availability rating is R or - let them have it (if they can afford it).
By 2000 everybody is using everybody else's equipment anyway, and there's troops of all sorts of different nationalities mixed in all over the place.
Might be worth pointing out there's a higher risk of friendly fire though...
http://forum.juhlin.com/showthread.php?t=2599

I tend to determine such questions (for anything beyond basic arms and a uniform, not just vehicles) by a roll against the availability percentage. To me, it represents whether the PC managed to come in contact with the item. The cost is a second issue....

Or, let the player tell you why he has such a vehicle. If it sounds good and suits your campaign, it's up to you, of course. Frank Chadwick will not come and check.

Uncle Ted

And that is basically how the availability ratings work, except during character generation.
Note also the table on page 26 of the 2.2 BYB
So it's quite possible for a group of players to roll up a BRDM-3, M2 Halftrack, or even Challenger II if the Ref so decides. Otherwise, if it's not listed as "R" or "-", and they happen to have sufficient cash laying about, there's not much to prevent them simply buying it.