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1.
When a man and woman (or another man) engage in the act of SEX. Half-way through their plundering, a Mexican midget jumps into the fun and stays between the two original fuck buddies for the rest of the ramming session.
Boy: Last night, I invited Pedro over for a Mexican Threeway. He was amazing. Too bad he was deported back to Mexico.
by DirtyLittleSpaniard July 04, 2011
 
2.
Where someone is preforming missionary position, whilst a canine is behind licking the person on tops anus.
Person 1: "Man i had the best night last night!"

Person 2: "I got you beat i had a Mexican threeway with Stephine and her dog!"

Person 1: "Gross man, just gross."
by Wiener/Cpt Decent December 03, 2010