There are a few way to get sum 3, thats mean u will be able summon 3 in same time. finish the quest or buying godstones.
– u can finish the quest by talking to zena first at market, she will tell u what u should do later.nevertheless u must  get two eudemons 15* atkdef.
the price for one 15* atkdef is between 600–750 ep depend on the seller.
-u also can sum 3 by buying godstone from the seller. each godstone is between 1.6k ep -2k ep.
if u want to get  sum 3 quickly ,u can buy godstone but u must get a lot of eps.By sum 3 u can have more pot and very usefull to up level minor eudemons for your main eud.