Edit - Apologies for any duplicated info to above.......interupted whilst typing my post at similar time
The legal limit is 1.6mm across the centre 75% of the visble tread width throughout the entire outer circumference and with non measured but visible tread reamining on the outside shoulders each representing 12.5% of the width.................Bearing this in mind & assuming your condition report is acurate then 3 with 3mm min and one near new will not result in a tyre related MOT fail in repect of tread depth. Do make sure you are checking the tyres properly though as it is possiblle for tyres to be under the limit in one area of tread pattern but to still have plenty of tread elsewhere.
On Diamaris at moment & since owned in 2006.......... If I was looking to buy one 19" tyre for all year round I would consider General Grabber UHP tyres. You will find mixed reviews but same can be said for other tyres. Tread pattern a little meatier than the Diamaris plus M&S rating - should cope a little better in winter and less than perfect conditions. These are all season tyres not dedicated winter/snow. Trade off's - Maybe expect a little more noise, possibly slightly lower mpg.
Shame that Toyo STII aren't available in UK- they are an all year round tyre & with special snipes to help cope with light snow. Toyo ST are favoured by many but I have seen recent discussions re recent low availability of these. Staggered 19" tyre sizes & limitations on UK tyre choices restrict options.
If you have 1 new Diamaris I suspect any saving in brand change to buy 4 matching other brand tyres plus fitting will be negligible when you offset that you only need 3 Diamaris.
Recently some have discussed merits of fitting winter tyres all year round rather than summers all year round. The logic based on the few weeks of good weather we seem to get. Ideally a summer tyre should be perform & wear better in summer than winter tyres, however, some summer tyres can also wear quickly too & even in fine weather.
HTH
