You'll find that the majority of cut rifled barrels such as Krieger and Bartlein are 4 groove barrels. The button rifled barrels will be where you see more or less grooves offered. Krieger and I'm sure Bartlein will make barrels with more grooves. I know Krieger offers 6 groove barrels, but you'll pay more for a 6 groove barrel than for a 4 groove. I've never bought or installed a cut rifled barrel that was anything but a 4 groove. All a button rifle maker has to do to make a certain number of grooves is have a button with that number of grooves and either pull or push it through the reamed hole that matches that button. To go from a 4 groove barrel to a 6 groove barrel, it takes a lot more passes of the cutter down the barrel to cut the extra two grooves. Every groove on a cut rifled barrel is a large number of passes of the cutter down the barrel. A 3 groove barrel would take a wider cutter and probably a lot of retooling. But, when you can sell as many of the 4 groove barrels as you can make and shooters win with them, there's not much incentive to make 3 groove barrels.
Jackie, if you want to see a barrel that's hard to indicate in, try a 9 groove barrel. I had some Harold Broughton .22 barrels that were 9 groove. Tim North still has the button and will make them. I chambered the first barrel and won the 100, 200 and grand with it at the first match I shot it at Luther, then the next month just about did the same thing winning the 200 and grand. I thought I had stumbled on the holy grail. The second barrel never shot as well as the first. Both barrels were made at the same time, both out of the same lot of steel, both were heat treated at the same time, one shot and the other didn't.