if you're constantly tired while on keto (and on the 3rd week even), perhaps your body did not adapt to it.

From my experience, I've seen people who "claimed" they're doing keto, but they're essentially just on low carb diet (big difference there). majority of people totally underestimated the amount of carbs you can consume while on keto. If you're just eyeballing your food and not knowing the macros (carbs, carb to fiber ratio), then most likely you're not even close to getting keto adapted.

But if you did everything correctly, then your body may needed more time as from what I've read, some people make take even up to 4-5 weeks before getting keto adapted.