As others have indicated, you're free to leave what you like to whom you like. (As Wak pointed out, a joint tenant can't leave his part of a property to anyone, simply because he doesn't actually own a specific share of the property. It's owned by the legal partnership between the joint tenants. However that's not relevant here).
So there's absolutely no reason why you can't divide your estate equally between your wife and each of your children. The only caveat is that your wife (or one of your children) could make an application to the courts (for a variation in the distribution of your estate) if they believed that your will had made insufficient financial provision for them. If you left absolutely nothing to your wife, such an application might well succeed but an 'equal shares' arrangement is perfectly normal and it's extremely unlikely that a court would vary the distribution of your estate:
http://www.statutelaw...eNumber=1&SortAlpha=0
Chris