It won't hurt them. 3 to 5 weeks is a common age to see coccidiosis. Common symptoms are lethargy, sitting fluffed up, not eating or drinking well, runny, mucousy or bloody droppings. The sneezing could indicate a respiratory infection, or it could just be from dust from bedding, or feed. Do they have any discharge from eyes, nares, beaks, any facial swelling, any discharge from, or bubbles in the eyes?
The correct dosing for the Corid powder is 1 1/2 tsp per gallon of water, for 5 - 7 days. If the chick is not drinking normally you may have to give it some by dropper until it starts drinking on it's own.
If the symptoms don't match and it's just having balance issues, then I would instead give it some B vitamins (a human B complex tablet or capsule, 1/4 to 1/2 of one daily). Do not give the vitamins at the same time as Corid, if you supplement thiamine while treating with Corid it can reduce the effectiveness of the Corid. B deficiencies are not uncommon and can cause balance and neurological symptoms. If you suspect coccidiosis then treat for that first, and give vitamins once treatment is complete.