A shower valve and a diverter built into a tub spout typically has both the shower port and the tub port both open at the same time. Because they make the tub spout outlet larger and it's lower, water falls out there rather than going out to the showerhead. WHen you close off the tub spout with the diverter, the water goes up to the showerhead because it can't get to the spout's outlet.
A diverter valve in your valve assembly MAY literally close off one outlet or the other, so in one position (when it's working properly) there's no path to both outlets. Having it part way in the middle might work, but that also may mean the volume available to the tub spout may not be full, so filling the tub would take longer.
Your best bet is to just repair what you have.
Well, really, if it's a 3-handle setup without scald protection, you best bet would be to replace the thing with one that meets current safety codes, but you are allowed to repair or replace the worn out part. You can't replace the whole valve with a new one, though, then, you'd have to bring it up to current codes.