Good Morning Sharol, 

Thank you for contacting the U of M Extension service. 

Your shrub may be reacting to the heavy rain we've had lately. 

I would check the stem where you planted them to make sure there is solid soil around the stem. Pull the mulch a few inches away from the stem. The wood chip mulch is good to keep the soil moist, protect the roots and keep the weeds down. 

Are the three shrubs planted near each other? Is the sun exposure different for the one that is turning yellow?

Please click on the following link that discusses shrub care. 

Here is another link about your shrub: https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=250190&isprofile=1&gen=Spiraea

Good Luck!