For new trees and shrubs, University of Minnesota Extension describes a 2–3 inch mulch layer and advises keeping it clear of the trunk. Plant type, existing cover and site conditions matter.
Measure what is already there
If you want 3 inches in total and have 2 inches of usable existing mulch, estimate the additional 1 inch. The calculator depth is the layer you are adding, not the intended combined depth.
Keep trunks clear
Leave space between mulch and tree trunks. Adjust the area you enter to exclude surfaces you will leave uncovered. Avoid accumulating deep piles with repeated applications.
Source: University of Minnesota Extension.
General guidance: adapt tasks to your local climate, plants and soil. Confirm product specifications with your supplier.