Circuit for driving a 5 wat led via 18650 battery
You'll need a LED driver, probably a buck (no need for a more complicated buck-boost if you accept some dimming when the battery is close to discharged).

Try googling "flashlight LED driver", there are many such products, which are usually in a round form factor which corresponds to the diameter of a 18650 battery. Here is an example. 



There are tons of choices on ebay, aliexpress, etc. If you want reviews and tests, googling "flashlight LED driver reviews" will give you some interesting tests, for example:

https://lygte-info.dk/info/indexLedDrivers%20UK.html

You can also look at the forum for these things, go into the DIY section and you'll find plenty of advice.

Also at 5W it will be a little brighter than at 3W, but not that much (our eyes do not perceive light linearly, they work more in a log-scale). The lower run time is not always a good compromise. 1-3W is a nice power, unless you want to go all out of course.