I think you would have time for those 3 parks. You don't need more than 2 nights at most for the crater, 2-3 for Manyara and Tarangire each. The migration will not be in those areas - you'd need to be in the Serengeti and earlier. By mid Sept most will be in Masai Mara in Kenya, although some stragglers might still be in the northern Serengeti.

Replied to your thread on Trip Advisor.

Here's my suggestion, by day:
1 - Arusha to Tarangire. Afternoon game drive.
2 - Tarangire - Full day in park (take picnic lunch)
3 - Tarangire - Ngorongoro Sopa Lodge on Crater rim
(Could visit a Maasai village near Tarangire or school or orphanage near Karatu on the way)
4 - Ngorongoro Crater (half day or full day if allowed)
5 - Backtrack to airstrip above Lake Manyara and fly to Kogatende airstrip in northern Serengeti. Stay in Lamai Wedge area at a camp such as Serengeti Mara Camp. While many of the Migration herds have moved into Kenya, the Lemai Wedge has substantial numbers that are resident year-round. Also, if the rains start early as they did on my trip in 2008, the herds will be moving back into the northern Serengeti.
8 - Fly back to Arusha

Skip Lake Manyara; you'll have much better game viewing in the other areas. Researchers say that up to half of the Migration herds never leave Tanzania, so you have a good chance of seeing them in the Lemai Wedge area. The Mara River flows through that area. Who knows? You might even see a crossing!

If you have not booked your overseas flights your should consider flying directly to JRO, rather than via Nairobi.

I agree with ShayTay on Lake Manyara, it sounds wonderful but really lacking in wildlife, if that's your prime objective here.

Yes it's pretty however a drive thru to somewhere else is really all that's needed.

ShayTay -
FYI - Serengeti Mara in Lamai is taking on a 'second' name - Olakira Lamai... who can keep up?

Only issue with the wedge is possible rain as these past season when the bridge was out and visitors couldn't get to/from. Unless the bridge is repaired/raised or whatever... can be a problem.

Thanks, Sandi. Did Asilia buy the camp from Afrika Afrika? Nothing is showing on the internet about the change.

No, it wasn't sold per se. Each are selling the camp under the individual names: Olakira Lamai by Asilia and Serengeti Mara by ArikaAfrika. At least for the time being.

Thanks a lot for your suggestions, Sahay Tay, take your point about Manyara. LeslieC, we have to be in Nairobi on work, hence can't fly straight to JRO. Am now considering doing a couple of nights at Masai Mara and then crossing the border into TZ, will keep coming back for your input, thanks again!