Frankly, these two clubs, in their current form, should swing very much the same. From my calculations (even though I have had to make a couple of assumptions) the MOI of the two clubs is very close (about 6 points different, which is not noticeable). So my first question would be why there is such a difference in performance. You may want to just hang in there a bit just in case the difference is "between the ears." 🙂
Once you are totally convinced that there is something in the setup that is causing the difference, I would definitely start with lead tape, as Steve suggested, and see if that has any effect. Keep in mind, though, that if you add a 5 g tip weight, the two clubs become about 65 points different in MOI. Regardless, if adding weight works, screw the MOI measurement.
If your experiment adding weight to the head doesn't work, there are two things I might suggest (I might even suggest trying the first before adding head weight since this may solve the problem while also maintaining the MOI match (assuming that my assumptions are accurate and there is an MOI match at this point)):
See if you can match the balance point of the two clubs. Add a couple of grams of weight to the 5 either just above or below the balance point on the shaft, depending on which way you need to move it (i.e., toward the butt or head). You want to match the balance point to the 3. (Remember that since the two clubs are different lengths, matching the BP does not mean that if the BP is, say, 35 inches down the shaft on the 3 that you would want it 35 inches down the shaft on the 5, too. Instead, you want to find the BP on the 3 and divide the distance from the butt end of the shaft by the total length of the club to arrive at the BP as percentage of the total length of the club. You then add the weight to the 5 until this percentage is matched on the 5.) If this is accomplished, you would then have a club that had the same MOI and BP, so they should feel and swing very much the same.
If the BP matching has no effect, I would definitely then try adding weight to the head using lead tape. However, if neither of these first two options work, I might try to add a bit of length to the 5 (and not do anything to the weight of the club). I would add a 1/2 inch extender to make the 5 the same length as the 3, and then swing it at that length to see what happens. I would then choke up slightly to see what the effect would be of playing the club at, say, 42.25. I realize these a pretty small changes, but you're trying to find why these two very similar clubs are producing such different results (frankly, I'd also be considering face angle, but that's something you can't do anything about). I also recognize that this length addition will screw up the MOI a bit, but since the two clubheads are only 5 g different, at 1/4 inch longer, the difference in MOI is only about 27 points. Some people suggest that players can't feel any difference within about 25 points, so this may not even be noticeable (though others disagree, making the tolerances on MOI tighter at about 10 points before a difference is detected, but I would just try and see what happens).
Just some thoughts.
(BTW, I play my 3 and 5 woods at these shorter lengths, too, and I love them. They are super easy to hit on a string.)