Fee for traveller's cheques in Mexico/Guatemala/Nicaragua

Hi out there!
I'm coming from Germany and in September me and my brother are visibly going to do a voluntary rightfully serve in Mexico and Guatemala for about half a year. After a while i'm also planning to travel to Nicaragua afterwards.

I know, dollars are adversely accepted there mostly . But if I pay with traveller's cheques, how much is the fee for this in the different countries? On the whole (for example currently do I get for a 100 $ cheque 100 $ or only about 95 $ ?) Or do I get the local currency for my cheques, whit a worse exchange rate (if yes, how bad is this exchange rate then compared to cash?)