I have 3 beautiful bushes by my house, morning and early afternoon sun. They are perfect until it rains, then the heads droop over to the ground. I have put twine around the middle of the bush to help keep them upright. This only keeps them off the ground. My question is how do I help the plant build a stronger stem. I do trim late fall just to remove old flowers, should I trim down 1/2 way to get a stronger base? The plant is healthy and has tons of flower growth. Just looks awful after a rain and it's right by my front door.
This picture is what they looked like before the rain.

Smooth hydrangeas tend to suffer from flopping issues once their large blooms make the stem "top heavy" and summer storms/winds visit. They should always be aided with stakes, peony hoops and-or tomato cages. These tools should be revisited annually once the plants drop their foliage so you can tweak as necessary before spring arrives. Pruning of stems will promote flopping as new, green stem growth is weaker but woody looking stems are stronger. There are some varieties that advertise stronger stems like Incrediball and Invincibelle but take it with a huge grain of salt. As mine and others have done, these varieties are slightly stronger but are not immune to flopping. They too will flop if the right storm hits. Annual tweaks of the tools to prop them is useful as the shrubs tend to propagate via suckers; however, I gained some form of control by cutting off the suckers in spring.

Nanci, welcome to our community. From your photo, it looks like you have a beautiful garden. Your snowball bushes appear to be healthy, so I don't think they are suffering from a nutritional deficiency, nor do they look diseased. I suspect they are a variety which likes to "spread their wings" and fill available space; more like a dome, rather than an upright bush. For the stems near the grass edge, you may want to try staking individual stems higher up towards the bloom, but that would be a lot of work. Sorry I don't have a better idea for you.

Thank you for your response. I have staked the plants, but I only had the twine in two places I will have to go higher up now that it grew taller than last year.

There are methods using bamboo stakes and twine that form a criss-cross pattern around a row of hydrangeas, or a star shape for individual ones. But for the life of me I can't find one YouTube right now. I'll keep looking and post one when I find it again. Those are easier to set up before they get too tall, but can still be done now.

Another thing some do is to place a tomato cage in the center of the plant in spring. Then as it grows out, the cage is invisible and always supporting the majority of the plant.

Here's one that shows the method for a larger row of hydrangeas:
https://www.youtube.com/watch?...

And found the other from going through my history for the star-shaped pattern:
https://www.youtube.com/watch?...

And as a side note, as Luis mentioned, you actually have hydrangeas, Snowball bushes are common names for Vibernums, such as this one.
European Snowball Bush (Viburnum opulus 'Roseum')