My recommendations would be:

Renaissance Sao Paulo - It is right on Paulista so if you only have a short time and want to be able to walk around a few interesting places, it can't get any better. Can be a little pricey, but compare that against taking an uber everywhere from places a bit further out.

Sheraton Rio de Janeiro - Splurge a little and have your beach experience here (actually the rates are usually quite reasonable especially now with the exchange rate.) You are very close to everything in South Zone plus have a very nice 'private-ish' beach and huge pool area when you just want to chill out without any 'public beach stress'. Ipanema or area around Sofitel/ Best Western will also be nice, but to be realistic as it is your first time in RIO, you will probably spend most of your time in an uber going to the various sights in other neighbourhoods rather than wondering the streets so why not stay somewhere memorable and special? Please do avoid hotels in Barra, simply too far from South Zone and Centro that time in traffic will just eat in to the time you have to enjoy Rio and you will surely miss out on much of the vibe and beauty on offer in Zona Sul.

Salvador - More difficult to recommend. I stay in Rio Vermelho (Pestana) which is a nicer beach area than Barra with some nice restaurants but a little far from the center. However the area and is a little dodgy, witnessed restaurants (plural) being robbed just outside the hotel on more than one occaision. Perhaps have a 'cultural' experience in Salvador, ditch the chain hotels and stay in a Pousada in the old part of the city for a few days. It is charming. Otherwise the Sheraton will for sure be very comfortable and OK located.

Iguaçu - You will spend a lot of your time in nature! So if you can, try and stay inside the park. I stayed cheap and cheerful in the city once. but maybe not so recommended unless you are familiar with Brazil already.. it has a 'wild wild west' vibe much like Ciudad del Este across the bridge ;-) I also stayed at the Mercure Iguazu on the Argentina side which was really great, right in the jungle surrounded by nature.