I will measure the size of the threshold. Then divide that amount to cut off in two. Then cut that one half amount off each end of the threshold so that the holes predrilled will be even and symetrical.
Then dry fit it and if its OK, then with a pencil mark the holes to the slab. Then get a good concrete drill bit and three inserts the same size. drill the holes, insert the inserts and then simply screw the threshold down. No problem.