Can I work under a car with jack stands?

I'll choose a set of 3 tons jack stands from a reputable brand, and
I'll buy an inexpensive floor jack. I mean, the floor jack is used while
we're not under the car usually. And, personnally it'll be used only
twice a year or so. Plus I'll be able to trust the jack stands while
working under the car. Do you have any advice on this matter?

First, it is never safe to work under a car on a jack stand. In the manual
of the jack stands from most manufacturers, mentioned
earlier, it's written: "Warning! Never crawl under the vehicle when supporting
it with Jack Stands". Don't take a chance with your saftey on a jack stand, no
matter how good it is.