Can anyone explain how to do a "3/8 turn combination" in the American Waltz? I can't seem to find out on the Internet. I wanted to know the basics of the step and how many measures it is. Thanks!

Yep, you can find it right on this website.

A video example can be found here:
https://www.ballroomdancers.com/Dances/media.asp?Dance=AWA&StepNum=107

There is no chart or technical breakdown of the entire combination, but there is of the individual components (e.g. Left Box Turn, Right Box Turn, and Closed Changes). American and Int'l style both use the same basic figures and technique, however currently on this website the American version is simplified. So you should choose the version you like not based on which style you are pursuing (as the patterns in both are identical), but on which description you find more useful on this website.

The simpler version is described in the American style here:

https://www.ballroomdancers.com/Dances/dance_overview.asp?Dance=AWA

The more involved version, which breaks the turn down step-by-step and also gives alignments, can be found here:

https://www.ballroomdancers.com/Dances/dance_overview.asp?Dance=WAL

For either, locate the Left Box (Reverse) Turn, Right Box (Natural Turn), and Closed Changes, then click on each respective icon for man's and lady's parts.

Once you understand how the individual figures work, connecting them is easy. The combination is as follows (from man's perspective):

Begin facing diagonal wall.

1-3 Right Box, RLR, with 3/8 turn to end backing line of dance.

4-6 Right Box, LRL, with 3/8 turn to end facing diagonal center.

Closed change forward, RLR.

1-3 Left Box, LRL, with 3/8 turn to end backing line of dance.

4-6 Left Box, RLR, with 3/8 turn to end facing diagonal wall.

Closed change forward, LRL.

Repeat.

This combination can also begin facing diagonal center with the Left Box Turn.

I hope this helps!

Regards,
Jonathan Atkinson
www.ballroomdancers.com