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[NickNack]

I would definitely replace all three units at the same time.  Eighteen years is a really old A/C.  The other units will go out soon, and you'll be sorry they all aren't the same.  

I would probably get at least two quotes, but I wouldn't necessarily go with the cheapest one.

[Fressa]

We have an older home that we purchased a while ago. We replaced the central air for the entire house. Last year we replace our boiler (hot water) system.  All of your units sound like they are at the age for replacement. We also have the maintainece contracts for both systems. I think that prolongs the life of the unit and provides a more efficient running. I have to say that if it were me, I would replace the entire system at once. It will cost you more to do it one system at a time and you might encounter complications once you start to marry individuals systems. We had to replace all the duct work as well but we have gained back in our energy bills. This air system runs so efficiently that it is paying for itself in the utility bill. The other benefit is that your house will only be under construction one time as opposed to three / fourr times. A big plus for me, especially since we have dogs.   

[CelticCrafter]

We don't have central air, we have a Daikin Ductless system that replaced a 25 year old Mitsubishi ductless.  

I am amazed at the difference it made in the electric bill - this summer it was half of what it was last year.

[ID2]

Wow OP how big is your house if you need 3 A/C units?

[ScrapHappy]

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@ID2 wrote:

Wow OP how big is your house if you need 3 A/C units?

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Almost 7500 square feet. 

[ScrapHappy]

Thanks everyone for your advice and input.  Looks like we have to bite the bullet.  But we're going to replace 2 A/C units and 2 furnances for now.  The other one (smallest one) is in the basement which is hardly run.  Unfortunately that too has it's own furnace ( a small one) so we will probably do that next year.  Can't do 3 A/C units and 3 furnances in one shot. 

[Snoopp]

I was told to replace the furnace and ac at the same time.  You don't have to do all 3 zones at the same time though.  I only replaced one zone.  The other is still fine.  The most important thing is you get a good company that knows what they are doing with the install.

[CAMOGIRL]

I would get a couple quotes.

When we had our a/c and heat system put in a couple years ago. I think we got 3. One guy was stupid, one was OK, and one my husband really liked him and what he had to say. Two quotes were about the same and one was ridiculous. 

Plus, you really should like the person and trust him to do a good job. I know some people look at the money, some people look at the person. I think both are important. I am surprised that some people are still in business quite frankly.

Basically - it does not matter how many people you want quotes from to find the one you want to use. I definitely just would not just get one though.