Working
Perixx PERIBOARD-804 (first-gen) BT keyboard.
I totally recommend this one if youre searching for a BT keyboard for less than $20/€20. It has rechargeable battery that lasts a month (with heavy use, it was my only keyboard) and the customer support is very responsive. My only gripe is that the keys are too small compared to other keyboards, but that's also the case for MS' overpriced Type Cover.
Leuchtturm1917 pen loop.
I attach it to the SP3, and it holds itself to the SP3 well.
Generic white miniDP-to-HDMI adapter. (€2.8)
It works with my 2560x1080 monitor well.
Partially working
Snakebyte idroid:con BT controller
It works only on ~10cm distance. If you use it further than that, there will be significant input lag. The issue also appears on my Acer W700 tablet, hence I would say the problem lies in the controller
EDIT: Seem to work better after I reinstalled the BT driver. Now investigating my W700.
Not working
BlackBerry Z10 (with the charger)
The USB port of SP3's charger couldn't charge my Z10 at all. It just keeps disabling and reenabling itself.