Despite the name, the T3($119) is the successor to the T4($109). Both are designed for IEMs. The T3 and T4 are the same size, which is to say TINY. No, it's really, really small. Like almost the size of a book of matches. I haven't heard the T4, but I like my T3. Most who have heard both state the the T3 is the superior sounding amp. Battery life is in the 35-40 hour range.
The D10($279, I believe) is a completely different beast. It's much larger, includes a DAC with the option to use USB, TOSlink, as well as digital coaxial I believe. The D10 also posses the ability to roll your own opamps to find the sound signature you prefer. The amp section seems to be pretty well regarded but is bested by the D4($219) which is also a DAC/amp combo but lacks the digital optical and digital coaxial inputs.
If you need a DAC and desire digital optical (TOSlink) or digital coaxial connectivity, the D10 is your only option from iBasso. If you can live with USB only, the D4 is probably a better buy.
If you want a small amp for IEMs and do not need a DAC section, the T3 is probably your amp. There is also a T3D with a digital pot, which is more expensive ($135, I believe), and per iBasso actually sounds slightly worse than the T3.
I wish they would have left their website up, as it would be nice to look at the stats of their products even if you can't place an order until after the holiday.