Higher in level would make them better if you want to get EXP along with the EVs.Boldore gives +1 Atk in addition to +1 Def.
The other 2 are inferior to the other options, which have the same EVs and are lower in level too.
Also, if higher level does make them worse, explain why there is an entry for the Baltoy that appear in Volcarona's room in Relic Castle. The Frillish in Virbank City are a superior option under that logic. Also, thanks for telling me that Boldore give +1 in both attack and defense, I thought they gave +2 in defense.