Answer: Unfortunately, I can't seem to come up with a combination of perennials that will do what you are asking in the sizes you seek and bloom at the same time or even have overlapping bloom times. For instance, you might find a red dianthus and a white dianthus in close to the sizes listed, but it would really depend on your microclimate whether or not you could find a blue to bloom along with them. You might try the early blooming veronica Heavenly Blue and allow it to sprawl so that it is low enough to match your design (however this plant blooms for just a short time in the spring and will not repeat) or perhaps try a salvia such as Blue Hill (which will usually repeat if deadheaded) and hope it begins to bloom early enough -- although it would be quite a bit taller than you indicated. Another possibility would be the shorter growing campanula "Blue Clips".
Using perennials in a bedding type of scheme is not usually very satisfactory because your planting will be colorless for much of the growing season; this is why annuals are normally used to create the designs. In addition, despite their "low maintenance" reputation, perennials still require attention including dead heading, dividing, watering, mulching, fall clean up and so on so the difference in work is maybe not as time saving as you are hoping. I'm sorry I can't provide you with a more satisfactory scheme, but maybe with some experimentation and luck you will find a combination that will do. Good luck with your project!
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