To do that you would just connect the output of the first curve to the inputs of the others, via the connection editor (accessible from the window : general editors menu).
The connection editor can also be popped up automatically by middle-dragging any node to another node in the hypergraph in graph mode (you will get a list of connection options via a context menu; chose 'other' to open the connection editor).
Now. Having said that, it doesn't really sound like you need 3 curves. If you just want to match values, you could just use one curve to drive all of the values. Connecting curves in series is the basis of the "set driven key" system, which is used to solve more difficult problems where there isn't a 1:1 correlation between the master value and the driven values.
Also note that if you DO connect curves manually, you might create some unitConversion nodes, depending on what type of input/output your curve nodes have. Like if you connect a unit output to a time input, it will create a unit to time conversion node in between. There are several types of animCurve node; animCurveTA, animCurveUU, etc. You might look them up in the Node and Attribute referece if you're interested in knowing more...
