You can compare them here, partly by looking at how much they cost new.
1995 Specialized Hardrock Sport - BikePedia
1995 Schwinn Frontier - BikePedia
1995 Trek 800 Sport - BikePedia

Unfortunately all three were entry level bikes with very low end components. They are not the old mid-level "gems" that people are looking for when they go out to refurbish an old bike. It would be well worth putting a set of inexpensive tires on them but not to spend a significant amount of money to upgrade any other components. If you can do the maintenance yourself, go for it. I'd put an inexpensive set of road or city tires on them unless you intend to go off road a whole lot.