Inviting another person into your bedroom isn't the way to spice up a marriage. If you have a solid marriage then you might decide to explore this option but you have to remember that this is another person with thoughts, feelings and desires that may or may not mesh with your own shared goals.
This isn't a 'tool' per se and should be used as one...if you feel your marriage is losing it's lustre then FIX the problem, after that it might be great to try some wilder things but NOT if your marriage needs some spice.
Threesomes don't, in and of themselves, destroy marriages but the mistaken impression that they are harmless fun does. Don't get me wrong threesomes and moresomes can be super fun and can enhance a couples shared experience, it can bring them closer and give them something to giggle about later. The problem is it WILL show you where the problems in your relationship are and unless you have a totally uninterested third you better hope your third cares enough about you that they don't wish to be a home wrecker. More times than not feelings are involved and not prepared for...this is where you get into the most trouble.