Naturally, the 3-legged stool is more stable and more difficult to knock over. Same applies to those who sell. Too many sales people rest on their laurels thinking they have a strong foot hold when they have an excellent relationship with their contact at their customer's organization. But what happens when that person leaves, or has a change of heart?
If your relationship with a customer rests on a single contact, you could easily be pushed out. While certainly enjoy the relationship, be sure to also cultivate additional relationships. Do this both vertically and horizontally, going high, deep and wide. This will give you a much stronger foot hold, as well as potentially lead to additional revenue opportunities. And the more people you know, the more referral potential as well.
I strongly urge you to review your client list and ask yourself, in which organizations is your relationship like a 1-legged stool and then identify steps can you begin take to convert those to more like a 3-legged stool. It's well worth the investment. Don't delay. Do today, as if your future depends on it, cause it very well may.
PS Similarly, actively strive to get more and more of your products and services into your customers organizations for this too makes it harder for a competitor to enter your space as well as for your customer to displace you. Moreover, it's easier to sell more to your existing clients then to get new ones.