grammar - if + should or if + present simple or ... Too many ways to say the same thing

What is the difference between the following sentences:

We have no jobs at present, but if the situation should change, we will contact you.

We have no jobs at present, but if the situation changes, we will contact you.

We have no jobs at present, but if the situation is to change, we will contact you.

Sentences 1 and 2 I took from Murphy, where they say the first one means that the situation is less likely to change.

Answer

There's very little difference between the first two, but the first is a little more tentative, and perhaps suggests that the speaker thinks it is not very likely to change, whereas the second is more neutral.

Having said that, the form with should is rather old-fashioned or literary, and many people would not say it at all.

The third form is different, and in this context incoherent. The is to construction refers to something which should, or is expected to, change. If the situation is to change, we will contact you means "We want it to change, and in order for that to happen we need to contact you". So it has a quite different meaning, that is not compatible with the first clause.