The ten-foot rule in 393.110 is a general requirement, that (as stated early in the section) is over-ruled by the regs for rolling equipment in 393.128 and .130. The 10-ft. rule is more to make sure there is a tiedown every ten feet or better on load like pipe, lumber, pallets . . . So a third restrain because of the length of the tractor is not a requirement, as long as the two or four restraints (depending on whether over/under 10K) is all that is required as a minimum.
That said and still . . . I went to the belt and suspenders school of you can't ever have too much for restraints. A little overkill can keep you safe in an extreme situation, and can reduce the aggravation if a diesel bear driving by thinks your securement might warrant pulling you over to see if he can't generate some revenue.
Nothing in the world wrong with tying down a 3000 lb. tractor at all four corners as if it weighed 13,000. Adequate straps, chains, hooks, binders . . . all marked for their ratings. All kept tight.