A THREE INCH water main (i.e. not a sewer pipe) in your backyard…? That’s a pretty big one for the water supply to a mere residence…even a three story appartment building. Is there something you’re not telling us, like you live on a farm with your own well, or you own a 15 unit appartment building?
As to you question, it all depends on the soil conditions and what kind of pipe it is and how it is laid. If the soil above and below the pipe is well-compacted and not sodden, you can pretty much roll a tank over it without a problem. Rigid supply pipe (like steel/iron) is less forgiving under deforming loads than more flexible pipes (like plastics…though you rarely see these in 3″ size).
Did you really mean a water main or a sewer pipe?