off topic a bit...
It really doesn't pay to use Cat3 wire anymore. I'd highly suggest you use Cat5 or Cat5e. We can buy it around here at less than $70/mft.
If the building ever wants to use multiple frequency DSL or other digital systems, the building is ready for them...
Yup, sure is and we see this butchered all the time.
Since you are used to using quad wire, hopefully you know that red is ring and green is tip of pair #1. If the quad wire actually has twisted pairs, you can use the yellow as ring and black as tip for a second pair. If the wire is the usual older stuff, where the pairs are not twisted, using both pairs for two lines will result in crosstalk between the two lines. The way to find out is to skin the outer jacket back a few inches and see if the red and green are twisted together and the black and yellow are twisted together.
The color coding you refer to has nothing to do with the wire being CAT 3. Instead of the solid-color color code of quad wire, it is a band-striped color code. In this scheme, there are 5 band/stripe colors and 5 binder colors which can be used to identify up to 25 pairs. It has been a telecom industry standard since the 1930's and all telecom installations follow this code.
Your 4 pair cable then follows this sequence:
Pair #1- white/blue= tip, blue/white= ring
Pair #2- white/orange= tip, orange/white= ring
Pair #3- white/green= tip, green/white= ring
Pair #4- white/brown= tip, brown/white= ring
As to using CAT 5 in an apartment building, I can't think of even one reason to do it. ADSL will work just fine on CAT 3 considering that the outside plant cable that brings it to the building isn't even that good.
CAT 5 and above requires a more stringent installation. You need to pay attention to bend radius, not deforming the cable with supports, no kinks, proper connectorization, etc. If you don't install CAT 5 and above properly you are wasting your time since it's not going to be any better than CAT 3, maybe worse. If your installation is simple POTS as in this case, it makes no sense to expend the additional effort and money on the installation and wire. Incidentally, the price difference between CAT 3 and CAT 5 plenum is quite significant as compared to PVC.
Respectfully, the notion to use CAT 5 for everything is usually born out of ignorance or inexperience. Using the proper cable for the job will make your job easier and make you more money.