I forgot that SDS3000X HD is quite a bit different when compared to the SDS2000X HD. The SDS3000X HD has 1 x 400 Mpts shared between all channels, while the SDS2000X HD has 2 x 200 Mpts, i.e. 200 Mpts per ADC. SDS7000A behaves the same, with the only difference that it is 2 x 1 Gpts instead.
The SDS2000X HD and SDS7000A behave like I've described, i.e. whenever both channels in an ADC are active, then the record length gets halved and the total memory remains constant.
SDS824X HD has shared memory 1 x 100 Mpts and acts as expected, i.e. a certain record length for single channel, half the record length for any two channels and a quarter of the record length for more than two channels.
For both architectures, total memory per ADC is always the same and number of History frames remains constant.
I can't check the SDS6000 right now, which has 1 x 500 Mpts and is therefore comparable to the SDS3000X HD. But even architectures with single memory are still different with regard to ADC: SDS800 has 2 x ADC and 1 x 100 Mpts, SDS3000X HD has 2 x ADC and 1 x 400 Mpts, SDS6000 has 4 x ADC and 1 x 500 Mpts.
I would expect the SDS3000X HD to behave exactly like the SDS800X HD - the screenshots above just don't show the true picture. The scope is still in Stop mode, hence any altered configuration cannot become effective. For each different setting (1, 2, 3, 4 channels active) you have to ensure that the scope is in Run mode, i.e. close the history first. Then you should be able to see the true record lengths. Furthermore, to be absolutely sure when checking the maximum possible number of frames, click on the "Frame No." field in the History and then on "Max" in the numeric input pad.