Even though our winters are generally much milder than in my youth, you still can't tell. I've been up the Ben in shorts and tee shirt in March and needed axe and crampons (although not on the trade route from Glen Nevis) in late April. Whatever the weather when you set off, it is foolish to go up the Ben without full body cover. My last Top
http://en.wikipedia.org/wiki/Munro) was on Liathach up in Torridon, which I went up on Midsummer's Day. At 2,500' it started to rain, which I had expected. At 2,800' it started to snow, which was a surprise.
Scafell and Snowdon are much less likely to have snow in April. I would still generally take at least lightweight full body cover, although both are less serious places than the Ben if things go wrong.
TBH unless you are very familiar with these hills I would always take map, whistle and compass, plus some rudimentary knowledge of how to use them. A light-coloured path across a boulder field becomes invisible surprisingly quickly in rain and low cloud and dozens of people get lost every year coming down the rounded upper slopes of the Ben: the wind whistles up the glen and in poor visibility inexperienced mountaineers instinctively trend downwind, which on that route can take you to places you really don't want to be.
In good weather the ascents will be a doddle, but in grim conditions do be wary of the "We've done the other two, so we can't possibly turn back now".
