Is there any difference between the following sentences?
- Please press the bell if necessary.
- Please press the bell as necessary.
- Please press the bell when necessary.
One can review use of if necessary, as necessary, and when necessary via examples drawn from links at ngrams. Some sets of examples are given below, with a summary after each set.
The above are typical examples of use of if necessary in explaining what action to take if certain conditions arise. It is supposed that the action might or might not be needed, depending on conditions. While as necessary could be used in these examples without particular change in their meanings, such use happens to be rare.
These two examples illustrate use of as necessary in cases where neither of if necessary or when necessary are quite right; specifically, where multiple actions may or may not be done, each dependent on conditions. Note that among numerous surveyed examples for as necessary, it is far more frequently used in a way not of interest here, in sentences like "skilled employees ... should be regarded as necessary employees" (Railway news).
In these and other examples, use of when necessary may be prompted by a sense that conditions will arise to make an action necessary; except that in the Kenya Gazette example, as and when necessary is, more or less, a verbose set phrase that could be adequately replaced by any of the simpler if, as or when phrases.
All three read roughly the same to me but if I were to divide them I would do along a spectrum of how often it may be necessary to press the bell. 3 implies (very weakly) pressing once, otherwise whenever would be there in formal language. 2 Implies several presses to my mind. 1 Seems very neutral. Alas I have no source or proof for these gut reactions so don't base any importance on my musings!