Conifers do not like wet feet. That could certainly be the reason for its demise. Also, it could pot bound. For 3 years the roots grow round and round the pot, getting bigger until it strangles the tree - root girdling. It may time to re-pot & be sure they have good drainage. Keep moist,but don't drown the trees.

I suspect the conifer has outgrown its pot and needs repotting, in addition have you given the soil any feed over the past 3 years?