Not familiar with your motor but have changed impellers on 6HP and larger Evinrudes. On those motors there is a small access plate on the lower unit where you disconnect the shift rod which stays connected to the upper part of the motor. Not sure if the 3HP has that plate? The copper water tube is tricky to get back into the block. On mine there was an "o" ring in the block that kept falling out. I finally put some gasket sealer on it to keep it in place. Turning the motor upside down might help, at least you can see better. The driveshaft should go in once you turn it a little each way to line up the splines. The water tube was the most difficult part. Maybe an extra set of hands might help?