There are common words you can say to yourself over and over that mimic the rhythm of various time signatures. If I remember correctly, for 3/4 time, it's "pineapple". If you imagine a waltz (or actually waltz...why not!) saying to yourself "pineapple, pineapple, pineapple...", you'll get the idea. Mute the strings on your guitar and just strum to mimic the rhythm, emphasizing the one beat..."PINE-a-pple, PINE-a-pple, PINE-a-pple...".

If you do this with evenly spaced up and down strokes, it may feel weird because of 3 being an odd number. If you strum up and down consistently, the first time around the ONE beat is a down strum, next time around it is on an upstrum...

D-u-d-U-d-u-D-u-d-U-d-u...

Using a down strum for the ONE and up strums for the two and three beats my feel more natural...

D-u-u-D-u-u-D-u-u-D-u-u

There are some clever words used for weird time signatures like 5/4 (Dave Brubeck's "Take Five" is the classic example of this time signature).

Unfortunately a quick google isn't turning up the full list.