A quick check with WinISD - 3.8 cu.ft / 25hz / single 3"
Your effective length of 5.43" is what you get if you flange both ends.
The normal mounting method is to leave the inside end free, so you select "One Flanged end" for the end correction. This gives you a required length of 5.79"
Another check is to use the calculator on the PSP website
It gives a "Straight port length required" of 5.83" - they must be using a slightly different equation to what winISD uses.
Now adding the adjustment for flares to find the overall physical length:
WinISD plus 1/2 flare radius gives 5.79 + 1.19 = 6.98
PSP website gives "Flare length port required" = 6.83"
PSP are adding 2 * 1/2" for their flare adjustment, which is what you would normally use for a 1" flare radius. I know that their new 6" ports use a 1" radius, so perhaps they have dropped their other products to that size as well - it would be interesting to get some feedback from people who have recently bought their products.
If that is the case then the WinISD method gives:
WinISD plus 1/2 flare radius gives 5.79 + 1 = 6.79 .... essentially the same as the PSP method.
The Installation Instructions on the PSP website for 3" ports suggests that each flare and connecting ring adds 3" to the length. This gives you somewhere between 0.98" and 0.83 for your straight section.
They suggest using ordinary ABS adhesive or similar type glue