designing sub enclosure?

Not if all the subs are inverted.  You can reverse polarity to the set by switching + and - at the amp speaker terminals.  This is something everybody should experiment with when setting up a system, whether the subs are normal or inverted.  Choose the way which makes the subs louder and deeper.  Some amplifiers have a "180" switch for this very reason.

Only if you had a setup where two of the subs faced one way and one faced the other way would you need to do some special reverse wiring on the subs themselves.

Here's an example of one sub wired in reverse of the other two subs.  This example shows DVC subs, but the concept remains the same with SVCs.  It doesn't matter if one sub is inverted and the other two normal...or one normal and the other two inverted.  You would wire the set this way in either case, because you can always reverse polarity at the amp.  Always find the simplest way to wire the subs knowing you have that 'reverse at the amp' option.  This is series / parallel.  If these were DVC 4 ohm subs the net result would be 2.67 ohms.

What is the purpose of the plexi?  if it's just a decorative covering, double stick tape, screws, or spray adhesive will do.  If it's structural IE, a window, screws every 2 inches + silicone caulking will do.