I looked at the manual, and there are perhaps a half dozen styles toward the end of the list which are classified as waltzes, so they're presumably in 3/4 time.
There are also a few styles with "6/8" in their names, and you might be able to use them to fake a 3/4 time-- but the waltz styles would be a better bet.
As for loading new styles, the PSR-E3xx series doesn't include a USB TO DEVICE port for inserting a USB flash drive, but if you use its USB TO HOST port to connect to a Windows computer or iOS device then you can use Yamaha's Musicsoft Downloader software (in Windows) or MusicSoft Manager app (in iOS) to transfer new style files into the keyboard's internal storage memory. After doing so, you'll need to use the StyleReg function to "register" (load) the desired style file as one of the 10 "user styles"-- Styles 206 through 215.
So if you can't find a built-in 3/4 style that suits your needs, you can look for more 3/4 style files on sites such as PSR Tutorial-- although you will very likely need to convert many of them from SFF2 files to SFF1 files, you might need to rearrange their variations to move the desired ones to A and B and to remove any unusable C and D variations, and you'll almost certainly want to revoice them so they use the best available voices on your model. There should be a number of old threads in these forums which talk about how to do those things.