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.

3 dogs in a bathtub by Slime Church March 11, 2011