There is a way in which you can bounce around between 3/4 and 4/4 without even having to change the time signature. The idea is to keep the 4/4 time signature over your song and whenever you have a section which would require a 3/4 time signature, you could use triplets to create a 3/4 feel to your song. 3 eighth note triplets equate to a quarter note duration in such a scenario. At least that’s how I’d to things, it all depends on your song though.
Also, if I may interject, what Ben is saying is correct, 3/4 and 6/8 are two different time sigantures.
If I remmeber my musical theory correctly, the difference between 3/4 and 6/8 is the time unit which is used to measure a beat in each measure/bar. For 3/4, it is read as “three quarter notes”, which means that the main duration unit is the quarter note and it is counted as 1 2 3, 1 2 3 etc.
For 6/8, it is read as “six eighth notes”, which means that the main duration unit is the eighth note and you actually have to count it as 1 2 3 4 5 6. 1 2 3 4 5 6 etc.
Though mathematically the bars using these time signatures last the exact same amount of time, the difference is in how you are supposed to count them. I remember my music teacher trying to convince me to use the correct counting method each and every time I had to play a 6/8 piece on a piano.
On a side note, I know a lot of music theory. I played piano for like 10 years when I was little then started playing the guitar and have since delved into a lot of music theory in order to understand how music works. If people find it useful, I can make topics tackling certain subjects when I have time.