Hawthorn tree dying?

I have three hawthorn trees together spaced approx 3ft apart and last year one of the trees (middle one) started to look odd and the leaves dried out & curled up late summer whilst the other two were fresh & green looking.  We left it in to see if it would come back okay this year & the other two have started to leaf but this one has just one branch with green leaves on & even those look to be withering up.  All the other branches appear to be dead as they snap easy.  There is some sort of green hardish mossy looking fungus on all three trees so don't know if this is anything to do with it or not.  Am worried that the other 2 trees may go same way.  Do I chop this one down?  Or will it come back again?  Anyone any ideas?  The trees have been in approx 3 years & are around 7ft tall.  Thanks.