3 hours is a bit soon for 40 flap landings in a C-172. Start with 20 flap and when you have that working, move on to a dozen landings with 30; then move on to 40.
Having to first flare against an upslope and then convert that to fit the subsequent downslope is a bit of a trick when learning a new type. You may want to visit an easier runway for touch and goes until you are better acquainted with the C-172. In my own case, I checked out in a Citabria on a 1900' strip with a downslope and dropoffs on both ends this Spring. For touch and goes, I'm taking it to a 5000' x 100' runway for the time being.
Back to the C-172 vs. the C-152. The biggest difference is that in the C-152, you were flying close to gross weight. One up in a C-172 is a significantly smaller percentage of gross than you have experienced in a C-152. The book approach speed for full gross does lead to a bunch of float when solo. Not having people in the rear seats puts you in more of a forward CG condition than you have experienced in the C-152 which means you need more pitch force to flare.
40 flaps requires a bunch more nose down to maintain airspeed; so, you have a longer way to rotate in the flare against the aforementioned forward CG condition. When you do get to 40 flap approaches, remember to get the trim set well before the flare.
Once you are used to it, you will find that the C-172 flies a power off approach with 40 flaps just fine as long as you do not put on full flaps too soon
Don't forget the carb heat
