Be warned though.
If you split your materia into three stats, you will not gain full bonus when upgrading to Nexus.

For example, my Novus was Det 18, Crit 24 and Acc 11 (to fill the rest), because I didn't want to buy several rank IV materias.

When I completed my Nexus, the result was Det 18, Crit 25 and Acc 11. So I've only gained 1 point whereas Nexus can gain up to 3 (No bonus point on Shield Nexus). I don't know yet the bonus you'll het for Excalibut but it already bugs me to "lose" some stat points.