The fact that you are still thinking about this kiss three years later suggests that there would be benefit from being open with your husband about this. It's not so much that he needs to know, as that as long as you keep it to yourself you are keeping something of emotional importance closed to your partner. Yes, it *might* be a needless risk to share this with him. You don't know how he is going to respond (or maybe you do, and you are afraid of the response!).
But the fact is, anything that distances you from your partner emotionally is an invitation to more secrets and more distance.
I can tell you with 100% confidence that IF you tell him, and IF he responds with forgiveness and compassion, your relationship will be much stronger and more intimate and more beautiful. But that latter one is a big and scary IF, and there is no guarantee that he will respond that way. However, IF he does not respond with forgiveness and compassion, that suggests that there is already a deeper problem in the relationship, and this revelation may open the door to seeking out healing for that problem.
It's a conundrum, but at least consider this:
1. You tell him, he forgives you, you can both let it go and deepen your openness, honesty, and intimacy together. [-0/+100]
2. You tell him, he has a really hard time with it, you go to counselling and uncover deeper issues, deal with them, and your marriage grows in maturity towards full openness, honesty, and intimacy. [-50/+100]
3. You tell him, he really can't get over it and one or both of you aren't willing to do the work to heal that rift. You stay in the marriage, but now it's worse. [-100/-50]
4. You tell him, he can't really get over it. The marriage dissolves, and you are free to find a partner with whom you CAN be wholly open. [-100/+100]
5. You don't tell him. He doesn't have the opportunity to forgive you. You continue to fret about it. There continues to be an unnamed/unacknowledged distance between you. [-0/-50]
The scorings are for the immediate and long-term consequences of your choice. By my accounting, there is a strong chance of some short term pain, but MUCH more possibility of long term benefit by being fully open and honest with your husband. And note that even in scenario 3, staying in an unhappy situation is still an outcome over which you have further control!
Hope this helps.