Rashi on Exodus
מלאו ידכם CONSECRATE YOURSELVES — You who are killing them (your own relatives) will by this very act install yourselves as priests of the Omnipresent God,
Sforno on Exodus
'מלאו ידכם היום לה, acquire for yourselves some degree of perfection today! Today you will become worthy to perform service in G’d’s Temple in the future!
Or HaChaim on Exodus
מלאו ידכם היום, "consecrate yourselves this day, etc." It is not clear what the word "consecrate" is supposed to mean in this context. If it referred to the matter of killing one's relatives, this had already occurred. It should have been reported prior to the Levites carrying out Moses' orders.