After remodeling my parent's old house - circa 1970 - I am trying to re-install the phone jacks in each of the three bedrooms. There is only one main line, so these are all extensions. All I have to work with is a line coming out of the wall and it contains a yellow, red, and green wire. The jacks I've bought however, have these colors, plus blue, orange, and white. What do I connect where? Do all 3 of the wires (yellow, green, and red) have to be connected to something? Thanks!

No. Green and Red should be a phone line. There should have been a black wire in the cable, but if there isn't, there isn't.

I have no idea where the blue, orange and white are coming from unless you have:
Blue with white,
White with blue,
Orange with white,
White with orange,
Green with white
And
White with green

Phone-man's Home Phone Wiring Advice Page - Wire Color Codes

RJ-11/RJ-14 Color Codes and Wiring AT&T Southeast Forum FAQ - dslreports.com

Only two wires are needed for a phone to function. Some phones are polarity sensitive especially ones that arephone line powered. No damage will occur if wired with the wrong polarity. Usually the phone won't dial.

There can be up to 6 wires in a typical jack.

I don't know if you accidentally bought 8 conductor jacks. RJ45 Wiring Pinouts and Hints
Lets not go here, if we don't have to.

The quad wiring (red, green, black and yellow) cannot be used with 110 punch down terminals.

Post a pic of your jack, front and back if you need more help. Use go advanced/manage attachemnts.

In any event, the two middle terminals are the phone line. Phone cords (wall to phone) are wired with a twist where the pairs are swapped.

A voltmeter can help. Looking in the NID where the telephone enters the house can help too.

As an FYI on the existing cable:

Green is "Tip"
Red is "Ring"
Yellow is "Bell" This circuit used to be used for resistance sensing so "Ma Bell" would know if there were illegal extensions in use.
Black - ground.

The coding was modified to allow for two separate phone line.

(Pair 1) Green = T1. Red = R1.
(Pair 2) Black = T2. Yellow = R2.