An output may be shared between any number of inputs up until the net effect degrades the output signal. Even a single 1:1 ratio of output to input can degrade the output. Its the net effect not the number of nodes that matter. As long and the following is within spec:
- Frequency
- Ripple
- Drive current
- Current direction
- Impedance
Now as for 1 clock to only 2 to 3 inputs, the answer is yes. This is done religiously and is called busing. If you have a scope, just monitor the signal of a single load, then add input loads making sure the signal is still strong. The weaker the driving output and the heavier the input load, the more likely the need for an intermediate buffer or increased buffer power. But since a buffer is just an output itself, it is still susceptible to the same degradation of signal.