When you're designing a trifold brochure, you have to put your contact information someplace on the brochure. The only question is, where?
There are a few options:
On the front
Whether you put the contact info on the front depends on your customers' level of familiarity with you.
If your brochure is meant for customers who are already familiar with you, and just need ordering information (like a take-out menu or spa menu), then the front is the best place for the contact info – that way, they can get what they're looking for quickly.
If you're creating the brochure for new customers, then putting all your contact info on the front is probably a bit early in the "relationship" to be asking clients to contact you. If that's the case, then reserve the front for making your product or service look extra-attractive.
The inside of your brochure may have many opportunities for calls to action – bits of text where you ask your customers to contact you. If this is the case, then plan to include at least some contact info – your phone number, email or website – next to the call to action. You may want to include multiple contact methods near the call to action.
On The Back
This is my favorite place to put the contact info on a product or high-end service brochure. This is typically a panel that people look at after looking at the cover – at minimum – so it doesn't look like you're asking for the sale right off the bat. The customer will be warmed up by looking at the rest of the brochure before you tell them to get in touch with you.
Also, the back is easy enough to get to if people do want to contact you immediately. People can just flip the brochure over instead of wading through all the text and unfolding the pages if they just want to call you or find your website.
I typically recommend putting the contact information in the bottom third of this back panel. I also recommend including some sort of call to action above all the contact information – it can help to spur the reader into actually using the contact information to get in touch with you instead of setting the brochure aside or putting it in the recycling bin.