We live in a 3 level house. The laundry is in the basement. Every time I wash clothes I smell sewage on the upper 2 floors... what can this be?

It may be that the waste is not properly vented. I used to live in a house that had the laundry in the basement and the plumber had installed the air vent for the waste inside the house, rather than running a vent all the way to the roof. This vent was designed for this purpose - it had a one way valve that was supposed to let air in but not let fumes out - but we often had the sewer smell in the house when doing laundry. So check to see that the waste vent is properly routed outdoors.

Thanks for your response. Where would the waste vent be?

Where would the waste vent be? In the wall where it can't be seen. Unless this is a add on I don't believe you're not vented.
A more likely culprit would be the basement floor drain. Over time and not being used the trap seal will evaporate allowing sewer gas to escape every time your washer discharges. Try this. Pour a quart of cooking oil down the floor drain and I'll bet the next time you wash the "stinky" will be gone. Good luck, Tom

Also, it may be obvious, but if there is a basement bathroom that is hardly ever used the trap under the sink and/or shower may also have dried out, so try running some water into them.

Speedball1: why do you suggest using cooking oil, as opposed to plain water?

Speedball1: why do you suggest using cooking oil, as opposed to plain water?
Because water will evcapoeate, especially behind Air Conditioning and you have the same smelly problem. Cooking oil or antifreeze will not. Cheers, Tom