Enspelled Armor

Can I add a level 1 spell to the "rare" armour and cast it at 3rd level, instead of adding a level 3 spell?

Generally, things like scrolls and other items always have the baseline spell. Spells are not upcast for items. 
But, if you’re the DM, it doesn’t seem like it would be game breaking to do. Usually a 3rd level spell is going to be a better use of a slot than an upcast 1st level spell. So if it were my table, I’d say it was ok.

I would totally let that happen, but ask your DM.

Quote from RighthAndyman618 >>

Can I add a level 1 spell to the "rare" armour and cast it at 3rd level, instead of adding a level 3 spell?

 As Enspelled Armor say ''Bound into this armor is a level 8 spell or lower '' so a Rare version contain an Abjuration or Illusion spell of level 2 or 3. 

A Spells Cast from Items is cast at the lowest possible spell.