can u run three cards without risers in this mother board?

1x to 16x riser will arrive my country about 20 days.

and i think my motherboard has two x16 3.0 and one x16 2.0

if i use three of cards without risers it may give a problem?

i thought i can start with 3 cards so i can earn some coins till risers arrive here lol..

it is forbidden that putting 3.0 card to 2.0 slot?