Another question.  I'm considering getting 3" toppers for my 2 daughters, but I wonder if it would work on one of the mattress that has some sagging.  Have any of you done that? It's talalay, with a choice of soft, med. or firm. I'm thinking of getting medium.  Thanks.

No.  You can make a firm mattress softer with a topper but you can't make a soft (sagging) mattress firmer.

Overstock dot com has a Mattress Rejuvenator(think is was made by Serta $104 all sizes)  ... a firm 1.5-2 inches of memory foam over a 2" very firm block of base PU Foam which does a fairly good job of  "bridging" a softish or sagging mattress.  It worked fairly well on my kinda soft mattress but my 170 pounds and bad back needed a new firm mattress under my comfort layer.  If your beds aren't too bad, weight not too much;, I would gamble $104 on the Rejuvenator if they still have them.  Rated like 4.7 of 5 stars.  Good luck.