If you need to do continuous A/D conversions, you can set up a program loop counter that counts to 3. On the first pass, you change the ADC channel. On the second pass, you start the conversion. On the third pass, you read the result...

Doing things this way makes the A/D conversion take essentially zero program time. Just make sure that you program loop time is longer than the A/D needs between the steps. If not, you might have to choose different "trigger" values for the loop counter. For example: you might have to set-up the channel on loop #1, start the conversion on loop #6, and read the result on loop #12.

This technique doesn't work in all situations, but works very well most of the time.