Context :

Aims : The purpose of this study is to find transient X-ray sources in M 31 , and to investigate and classify their nature .

Methods : Three X-ray transients were observed with Swift .
For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst , and thereby to identify the source types .

Results : The outburst of XMMU J004215.8+411924 lasted for about one month .
The source had a hard power-law spectrum with a photon index of 1.6 .
It was previously identified as a Be/X-ray binary based on the optical identification with a star .
However , we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source .
The source SWIFT J004217.3+411532 had a bright outburst , after which it slowly decayed over half a year .
The spectrum was soft , corresponding to a thermal accretion disk with innermost temperature of \sim 250 - 600 eV .
The source was not seen in the optical , and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary .
M31N 2006-11a is a nova that was previously observed in the optical .
We detected it both in X-rays and UV with Swift \sim half a year after the optical maximum , after which it decayed below the Swift detection threshold within a month .
The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV .
We use catalogues of X-ray transients in M 31 to estimate their rate , and we find a lower limit of 9 yr ^ { -1 } .

Conclusions :