Make all phones on the same line?

We work at a small one-room store and have three phones spread throughout. As of right now, our 3 phones (two are handheld one is wired) all have different numbers configured to them. What we would like to do is just put all of the phones on the same line. Our old system made it easy to place someone on hold and then go pick up the other phone. Now, we have to forward the caller to one of the other phones with a full phone number. We just want all 3 phones on one line, or at least to be able to switch between lines on each phone. Do I need to speak directly to Comcast?

Thanks.