3 TO 2 NAP TRANSITION

Hello! My 7 month old refuses to do the 3rd Nap, but she sleeps for 1 hour each nap, how do I lengthen her naps so that she sleeps 3 hrs total a day? If she only sleeps 2 hrs total she is cranky at night, I put her to bed early for now and wait for her to lengthen?