Josh, lisa, and vicki each ate 1/4 of their own pizza.. each pizza was the same size, but josh ate 1 slice , lisa ate 2 slices and vicki ate 3 slices .. how is that possible

These three people each ate 1/4 of there own pizza
Josh= 1 slice
Lisa= 2 slices
Vicki= 3 slices

Josh ate 1 slice which was cut half way through. In other words, the pizza was cut into only 2 equal pieces. 

Lisa ate the pizza which was cut into  equal pieces and by eating 2 of those pieces, she ate 1/4 of a pizza. In other words, 2 of 2 is 1/4

Vicky ate a pizza which was cut into 2 equal pieces and by eating 2 pieces he ate 1/4 of the pizza. 
In other words it's 1/4 

Hope this helped! :)