Well the two outside ones look like air bleeding valves on 10mm pipe but the middle one looks like a short length of 10mm pipe.
I like KIAB's reply, however, yes, they are bleed valves in 15mm, lovingly hand crafted ready to be inserted into end-reduced T's on te feed and return to my UFH. Currently sitting at 2 bar to check for leaks and all looks good.
By far the easiest way considering the routing and that the UFH is plastic anyway! Hand crafted by ... Pollowick Enterprises. Available to you in either 15 or 22 mm at any length required!