Transformer with 3 secondary taps
Yes, it is possible to use more than one winding at the same time, but you have to be careful that you violate neither the power rating (18&nbsp;VA) nor the current rating (1&nbsp;A) at any of the four taps.

This means, as an example, that you can use the 12&nbsp;V and 15&nbsp;V taps if you need a voltage of 3&nbsp;V, but you can only use 1&nbsp;A at these taps, which equals to 3&nbsp;VA - not 18&nbsp;VA.

Also, there is an isolation between the primary and secondary windings, but the three secondary windings are not independent from each other (i.e. not isolated). Probably not a problem for you, but still something to keep in mind.

The data sheet has a circuit diagram of your transformer and includes the current rating (1&nbsp;A max).