There are many posts regarding the use of three- way switches. Let's go over a few of the basics:
*A circuit using three-ways MUST have TWO three-way switches.
*The two three-way switches are connected to each other by two conductors called travelers.
*If you try to use a single pole switch in conjunction with a three-way, the single pole switch will switch the load only if the three way is in the position that allows the conductor connected to the single pole to be energized.
*A three-way switch is not marked on/off like a single pole switch is, because it really is a case of this traveler conductor is energized or the other is. (Guess we could call them either/or switches?)
*Additional locations can be allowed switching by placing four-way swiches between the three-ways, but let's not go there.
*The basic use of three-way switches entails the hot wire from the breaker being attached to the common terminal of the first three-way, two conductors "traveling" between the switched terminals of that switch to those of the other three-way switch, and then a single conductor from the common terminal of the second three-way switch to the load. (See diagram, equal signs denote the travelers..)
*Depending on how a circuit is layed out, you may position a three-way switch to deenergize a fixture and still have an energized conductor in a switch or fixture box (the other traveler wire).
*Look at flea markets, yard sales, book stores, Goodwill or anywhere you can and find a copy of Time-Life Basic Wiring book. The pictures make this all very clear. Sorry for being so long-winded, but electrical (and gas) are things you want to be absolutely sure you understand what you are doing. Luck, Kizmet