Allow Tardiva 6-8' and space out evenly the rest.
Your problem down the road will be the width of the bed, 4' is barely wide enough for the macs and definitely is way TOO narrow for the Tardiva. Unless you want to butcher the plant every year.
However, if you'll train your Tardiva into a single trunk tree you might be able to keep its crown at 5-6' diameter.
Still, yearly pruning will be necessary.

Don't toss the tardiva. I love mine and hope you will love yours, too! Mine has a lovely open branching shape with several stems/trunks and it is in front of a fence with other shrubs on either side. Mine is about 10 years old and would fit fine in the space you are describing. I have never pruned it.

Tardiva is a Paniculata, and according to the link below, Unlike mopheads, they need several hours of sun to do well.

You might want to consider planting it somewhere other than your shade bed.

Sue

Here is a link that might be useful: Paniculata Hydrangeas

Thanks for the advice! I can give them all more room; the bed is fenced on one side, but only limited by grass and old railroad ties on the other side. I can put the Tardiva on the end that gets the most sun, then how would you arrange the other two; put them together or every other one? I have a hard time picturing which way will look better.
thanks again

Usually pairs are not the way to go. Could you get 3 of 1 and then have the singles at each end?

As someone mentioned the Tarvida will need plenty of sun...my paniculatas get 14 hours of sun during the summer.

cyn427,
'Mine is about 10 years old and would fit fine in the space you are describing. I have never pruned it.'

How it fits fine in 20' I could imagine easily, but '10 y.o. never-pruned-Tardiva' fitting FINE in 4' is beyond my imagination, sorry.
Probably it's time to check my measuring tape :-))