3 Days? Your leg will start to get ache during your descend from Senaru crater. Average walking step will be around 20-30k steps per day.

Day 1 - Start from Sembalun. Pretty easy walking on flat savannah terrain. Do not need to wear your wind breaker/ warm wear first as it is well exposes like Broga. Along the way do BEWARE OF COW DUNG. Plenty of them along the path and start to get lesser when you will make your stop at CP 2 (if not mistaken) for lunch near the bridge. Try not to eat near the edge as monkeys will try to snatch your food or belongings if not being careful. Wild monkeys are you will not get back the items. Things will go slighly difficult when ascend from CP 3. About 45-55 degree up for about 1.5 - 2.5 hours before reaching the campsite where you going to stay for that night.

Day 2 - You will wake up around 2-3 AM (very similar to Kinabalu) and start hiking to Rinjani peak. The last 1 km to the peak are all sandy and my best advice is to get a pair of trekking poll which could be useful for your end as every three steps you make you will fall a step backward. Once you reach the stone area then you are pretty ok and can enjoy the Rinjani summit. Do not spend too long up there as you will be using the same way to descend all the way to your campsite. Unpack and head down to a slippery small cliff which leads to the savannah before reaching the lake side about 1-2 hours from the Sembalun crater.

Your itinerary make things worst for you as you mention you will be camping at Senaru Crater on Day 2. If you are not prepare, things get pretty worst here. You could question your group leader on why did they want to camp at Senaru Crater instead of the lake. Was it because just for the sunrise? From lake to the crater might took up easily another 2-3 hours with rocky terrain all the way up up. I did my research earlier for my trip and I discard camping at Senaru crater and settle down near the lake. Nearby the lake there is a waterfall and a hot spring as we could enjoy a good rest before pushing the limit on the final day.

Day 3 - According to your plan which is the night in Senaru Crater, from there you will descend all the way downhill until you reach the hut area where more likely you will meet group starting climbing from Senaru gate instead. Your guide will more likely cook your lunch here and push down to the summit all the way until you reach the Senaru gate. Do not be too happy when you reach the Senaru gate as you will need to trek out another 0.5 -1 hour to the main road.

You could ask me more questions about Rinjani and I will assist if I know how to answer them.

Any other mountains which I would recommend? Depends what you are looking at. Oversea or local?

If Indonesia, most people will hike Ijen and Bromo first although I felt Ijen was not up to my expectation. Everyone got tricked by the blue lake which doesn't interest me much. Others might consider Kerinchi, Agong, Batur, Semeru and Merapi.

Local got a lot of nice mountains too like Stong, Yellow, Bubu, Tok Nenek, Tebu, and many more.