All these 3 plants need to thrive is well-draining soil in a decent amount of sun. The soil additions described are more suited for container gardening rather than amendments that would be particularly helpful for this situation. At most, just a bag or two of garden soil or topsoil can be used, but if drainage is good, not even that is necessary.

Gardengal48, thank you for your comment. I wondered if all these amendments might be excessive.

The thing is I need to add about 4" to what I have to get the soil level with the front rock wall. Can I just add 3 way soil (topsoil, sand compost) or 60/40 soil (sand compost) to the top of the wall and mix it in some? I will be getting the soil in bulk because the wall is long about 55'. It is in full sun. Right now drainage is pretty good.

Aubretia and Candytuft are very unfussy and will almost certainly grow in your native soil providing it isn't heavy and wet. Poor and rocky would suit them fine. Lithodora is a little more particular as it doesn't like lime. No need for expensive bought soil. Regarding weeds ensure any perennial weeds are well and truly gone before planting.