"Dram" or "Dram Equivalent" is an old term that equates the speed of a smokeless powder load to an equivalent load propelled by black powder. As such, there is no conversion factor between black and smokeless powder. In today's world, it is better to think in terms of the weight of shot and the speed at which it leaves the barrel.
The term "Dram" is deeply entrenched in the minds of many trap shooters and I like it also. I load for 1 1/8 oz 2 3/4 dram equivalent speed in many of my reloads which is 1145 feet per second. I try to stay at this speed whether I load 1 oz or 1 1/8 oz.
Here are the published DE's for 12 gauge 23/4" shells. Again, each entry is independent of the other and in the beginning were developed using black powder.
1 oz - 2 3/4 DE = 1180 fps
1 oz - 3 1/4 DE = 1290 fps
1 1/8 oz - 2 3/4 DE = 1145fps
1 1/8 oz - 3 DE = 1200fps
1 1/8 oz - 31/4 DE = 1255 fps
1 1/8 oz - 3 1/2 DE = 1300 fps
1 1/4 oz - 3 1/4 DE = 1220 fps
1 1/4 oz - 3 3/4 DE = 1330 fps
1 ½ oz - 3 3/4DE = 1260 fps.
As been said many times, always consult the proper loading manual and weigh your charges. Use of different components (i.e., different wads or primers) than those stated in the loading manual may result in different results.
I suggest that you always think in terms of speed of the shot charge and not DE. You mentioned a 3 DE 1 oz load. I don't think that there is a published speed for this loading.
Also, there are now "lite", "extra lite" and reduced recoil loads available and I believe that these loads show the speed of the shot and not the dram equivalent which does not exist.
Also, you may see the term "MAX" dram equivalent or "HDCP" dram equivalent which may have a different meaning from each of the various manufacturers.
If you stay with the speed of the load, you will have a sound basis for comparison.
Also, you may hear the terms speed and velocity used interchangeably. They basically each mean the same except that velocity has a direction associated with while speed does not. Example, for a vehicle traveling 50 miles per hour north, the speed is 50 miles per hour and the velocity is 50 miles per hour North.
However, if you're equipped with a chronograph and any skills, you can custom cook any load, weight, speed that turns your crank If you really like 1200fps for some reason, in other words, you can adjust your 1 oz loads, within safe limits, to match the 1 1/8 oz velocity.....Bob Dodd
The only thing to worry about is "too different" fast or slow as it screws up your perception. I do not consider myself a top level shooter, but even I see the effects of drastic changes in shell speeds. Whats drastic? Thats up to you. I would bet after 5 or 10 birds with any shell you get dialed in.
You said 3 dr 1 Oz and 3 Dr 1 Oz, is that what you meant? even if it was 1 oz and 1 1/8 it is about the same. Focus on your shooting as "all shells are better than all shooters"
A "Dram" is 27.34 grains. So a smokeless load labeled 3 dram eq (regardless of shot charge) is supposed to be the equivilant of that shot charge loaded over 3 drams (or 82 grains) of black powder. Now, after 100+ years of smokeless powder loads, comparing smokeless loads to black powder loads really doesn't equate. And as been discussed recently, a 12 ga 1 1/8 oz 3 dram eq load can be 1200 fps + or - 90 fps, and still be a 3 dram eq load.
We shouldn't even be talking in drams, but I think many non-target shooters would be just as ignorant if specific velocity being listed, as they are when dram eq is listed on a box of shells. In their world it starts and ends with high and low brass.