I think the people telling you not to share the message with your husband are being ridiculous. Let's face it: Just as there is no way for you to prove what he did or didn't do three years ago, equally there is no way for him to prove his innocence. The hotel won't have CCTV, and if they did, they're not likely to hand it over anyway.
The person who sent this vicious and nasty message is the villain here, and the intent of it is to split you up and hurt your marriage.
You know there is a person who worked with your husband who has a grievance against him. You know that that person would probably know his movements on work trips. The fact that a date three years ago is being mentioned is not a co-incidence. It's just long enough gone to stir doubts without ever being provable.
You have no evidence against your husband. You have the opinion of a good friend who says she thinks he'd never stray. You have nothing other than a malicious text message sent to you to provoke this exact reaction.
The best and smartest thing you could do is tell your husband, and to ignore the contents of the message.
You have no basis or evidence to do anything else. From what you say, his was not the reaction of a guilty man. Think for a moment what he is going through right now if he is innocent. Think of the distress that is being caused. Imagine if he came in home this evening and did the same thing to you and you were here posting a thread called "husband accused me of cheating out of the blue".
You are letting a third party damage your marriage and it is insane and the people here advising you to continue doing this are mad.