I haven't responded earlier as am not in your zone and don't deal with those hot dry temps often in our summers.

I'm not sure adding more plants to compete for water is a good idea. Some people add large rocks in front to give shade.

Hopefully others will respond.

As for companions do you want them to bloom at the same time? Do you want complimentary colors or contrasting colors?
Do you want small blooms or large blooms? Do you want a different shape? Some blooms are more open than others.
Answer those questions and then choose another group 3 clematis that is available in your area or if you don't mind waiting order online.

You might also look through some of the photos on this section and on the gallery section to see what others have done for inspiration.

Here is a link that might be useful: Spring Fever

This link is quite useful, thank you. I will try some rocks there, I have lots. And the link photos are very inspiring and make clear the way different varieties are in real life, not catalog glamor close ups.
My vines are continuing to grow so I suppose they are not getting too blasted by sun at least.
Pea

Here they say "Companion plants, such as this pale-purple baptisia, keep the clematis' root-zone cool."

http://www.gardeners.com/how-to/grow-clematis/8203.html

I hope that helps.

I don't know how you water, but have you considered a weaping type of soaker hose buried under the mulch so water is going directly to the roots rather than getting on the plant leaves and having to soak through the mulch? I am thinking of a ring or spiral of weapy hose like that in the link below. I am not recommending this particular company or product, but I have used a similar product in the past and found it works well to keep moisture higher in dry areas.

Here is a link that might be useful: Example of weaping soaker hose

You don't need rocks or companion planting.

They need to be planted deep and mulched heavily.

They hate root competition.

Some idiot wrote keep roots cool decades ago when he meant keep evenly moist and the mix up has persisted ever since....

Ok, this is sensible advice. I will add to their mulch level and just keep them in their own bed. Maybe people want to hide the bare knees of the clematis- as for me, I don't care, they are below eye level next to a service walk at the side of my porch and I only see them when I go water.
Thanks for helping, my gardening friends.
Pea

Bare knees can be taken care of with proper pruning in many cases....

Certainly you can plant in front of them to hide them as well but the plants need to be away from the root zones and not something deeply rooted or aggressive.