Draw ASCII boxes in boxes
Jelly, &nbsp;20&nbsp; 19 bytes

-1 byte using the quick ` to avoid a link, as suggested by Erik the Outgolfer.

H»þ`€Ḣ&gt;Ð€HSUṚm€0m0Y


A full program taking a list [a,b,c] printing the boxes using a:2 b:1 c:0
... in fact, as is, it will work for up to 10 boxes, where the innermost box is 0 (for example).

Try it online!

How?

H»þ`€Ḣ&gt;Ð€HSUṚm€0m0Y - Main link: list of boxes, B = [a, b, c]
H                   - halve B = [a/2, b/2, c/2]
    €               - for €ach:
   `                -   repeat left argument as the right argument of the dyadic operation:
  þ                 -     outer product with the dyadic operation:
 »                  -       maximum
                    - ... Note: implicit range building causes this to yield
                    -       [[max(1,1),max(1,2),...,max(1,n)],
                    -        [max(2,1),max(2,2),...,max(2,n)],
                    -        ...
                    -        [max(n,1),max(n,2),...,max(n,n)]]
                    -       for n in [a/2,b/2,c/2]
     Ḣ              - head (we only really want n=a/2 - an enumeration of a quadrant)
         H          - halve B = [a/2, b/2, c/2]
       Ð€           - map across right with dyadic operation:
      &gt;             -   is greater than?
                    - ...this yields three copies of the lower-right quadrant
                    -    with 0 if the location is within each box and 1 if not
          S         - sum ...yielding one with 0 for the innermost box, 1 for the next, ...
           U        - upend (reverse each) ...making it the lower-left
            Ṛ       - reverse ...making it the upper-right
             m€0    - reflect €ach row (mod-index, m, with right argument 0 reflects)
                m0  - reflect the rows ...now we have the whole thing with integers
                  Y - join with newlines ...making a mixed list of integers and characters
                    - implicit print - the representation of a mixed list is "smashed"