"I used this connector to join 2.5mm² 3 core Arctic flex and I was not impressed. The 3-pole terminal block internals of this 3 way connector are the same as the in-line TeeTube (Scewfix Product Code: 92911) which works very well, however you are connecting half as much again copper conductor (and thus Live, Return and Earth copper sheathing) with the 3 way without any allowance for this in its design, because, as I say, it uses the same 3-pole terminal block internals as the in-line Teetube (2 way) connector, which I would recommend.
If I had used it to connect 0.75mm² or 1.5mm² 3 core maybe it would have perform better, but I was not, and in the specification it says it can except 4mm² . Personally, I doubt this claim very much.
My requirements of this installation did not allow for the use of a Wiska IP66 / IP67 Weatherproof Junction Box, which Screwfix use to do, or Screwfix's current offering, the BG Nexus 4-Pole Junction Box, (Product Code: 33518). If you need to connect round 2.5mm² 3 core flex or higher in a 3 way configuration I suggest you try and use one of the aforementioned products rather than the 3 way Teetube IP68 3-Pole T-Splitter."