3 dogs in a bathtub
The act of inserting one's entire penis and scrotum into an anus or vagina. The difficulty of that feat is akin to getting three dogs to take a bath at once, hence the phrase 'three dogs in a bathtub.'
Fitting my whole package into Sarah was harder than putting 3 dogs in a bathtub.
Or:
Kelly was so wide open I put three dogs in the bathtub.
Or:
Kelly was so wide open I put three dogs in the bathtub.