The external hardware differences are reasonably clear.
1. the 3x25 has a 5v external power connector, or can be powered by USB. The 1025g is USB only.
2. the 3x25 has 5 BNC connections: OUTPUT, SYNC OUT, COUNT IN, CONTROL, TRIG IN. The 1025g has only 2 BNC connections: OUTPUT, SYNC OUT.
3. The 3x25 has 6 digital inputs, I 0 through I5. The 1025g has only 4 digital inputs, I 0 through I3 (of the 20 pin connector). The
advertising (and specifications page in the manual) claims the 1025g has 6 bit input, but the manual clearly shows it's only 4 bit.
4. The 3x25 also has 12 digital outputs, O 0 through O11. The 1025g has 12 digital outputs, O 0 through OB (of the 20 pin connector).
5. The 3x25 has two ground pins on the 20 pin connector, the 1025g has 1 ground pin.
6. If you have been doing the math thus far, that makes a total of 20 pins accounted for on the 3x25, but only 17 on the 1025g. The other 3 pins seem to correspond to the missing BNC connectors: CTRL, COUNT IN, AND TRIG.
7. The 3x25 has a second 20 pin connector labelled "MULTIPROCESSOR LINK" used to synchronize multiple 3x25 units for multiple signal outputs using the homebrew ribbon cable shown in the manual. There is a section of the manual titled "Combine Devices" outlining how to do this. The 1025g does not have this connector, and the manual does not have a "Combine Devices" section.
The 1025g
advertising claims "Standard USBXITM interface, easily inserts into USBXI housing to make up a combination instrument." However, this mysterious "USBXI housing" seems to be tradmarked vapourware. It's not listed anywhere in their product pages that I could find. Hantek's site search pulls up the 1025g and 3 pc-based scopes that claim to use it, but not the housing itself. Google produces similar results. As far as I can tell, the "USBXI housing" does not exist. I can only guess that Hantek meant to produce some sort of hub that would synchronize these products through their USB connectors but for some reason couldn't get it to work properly and gave up.
Just to be complete about this: Hantek doesn't seem to sell the "Optional Multiprocessor Link Cable" for the 3x25 either. However, they do at least show a couple of photos of it in the manual, so a standard IDC 20 pin cable can be modified quite easily:
Cut wires 1, 2, 3, 4 at each connector and leave them open.
Cut wires 17, 18 and 19 between the master and slave 0 connectors, connect all 3 together at the master connector and leave them open at the slave 0 connector.
Cut wire 16 between the master and slave 0 connectors. Connect wire 16 on the master connector to wire 15 on the slave 0 connector. Leave wire 15 open at the master and wire 16 open at the slave 0 connection.
Cut wires 16, 17, 18, 19 between slave 0 and slave 1. Connect 17 and 19 together at slave 1 connector. Leave all others open.
Cut wires 16, 17, 18, 19 between slave 1 and slave 2. Connect 18 and 19 together at slave 2 connector. Leave all others open.
NOTE: I only have one DDS 3x25, so I have never actually built and tested the above, I'm just going by the pictures! I'm
sure Hantek would
never make a mistake or omission in any of their documentation

, but
use at your own risk anyway.