I know there are 3 pacts: blade, tome, and chain, but are there any rules/ guidelines to make one that's none of the above?
You can homebrew your own pact
You are correct that there are currently only the three pacts available in officially published material. As with everything in D&D 5th edition, however, there is nothing to prevent you from creating your own.
Special Terms from XGtE
Xanathar's Guide to Everything added a lot of material for Warlocks. There is a section on "Special Terms of the Pact" where it describes what a pact is:
A pact can range from a loose agreement to a formal contract with lengthy, detailed clauses and lists of requirement. The terms of a pact - what a warlock must do to receive a patron's favor - are always dictated by the patron. On occasion, those terms include a special proviso that might seem odd or whimsical, but warlocks take these dictates as seriously as they do the other requirements of their pacts.
It goes on to provide a table of examples of special traits you can add to an existing pact to make it more unique. If flavour is all you are going for, then this could be enough. If you want a more custom pact than this we fall back onto the standard homebrew rules from the DMG.
Creating New Character Options from the DMG
There is a chapter (pg. 285) in the Dungeon Master's Guide dedicated to creating new character options. The particular section you are interested in is titled "Substituting Class Features" and begins on page 287. The full rules for this are too large to include in an answer here but I will give you this extract:
If one or more features of a given class don't exactly fit the theme or tone of your campaign, you can pull them out of the class and replace them with new ones. [...]
There's no formula that can tell you how to design new class features. The best place to start is by looking at other class features, or at spells, feats, or any other rules for inspiration.
Creating your own pact
I strongly suggest you read the full chapter mentioned above for advice on homebrew. Once you have an idea of what you want to make, try to write out the feature in the same style as the other pacts. Compare it in terms of power, uses, and overall balance. If you can, find a way to playtest your creation in different situations to check for issues. You can ask on forums for help if required.
Once you believe you have a good, balanced feature that you want to include in your game, you can ask a new question here as a homebrew review question. Please read the guidance on How can I ask a good homebrew review question? before doing so.1
1 Homebrew questions are currently being discussed on meta. See these metas for more information: