Llyn y Tri Greyenyn was a small lake not far from Cadair Idris in the old parish of Tal-y-llyn, Merionethshire (now Gwynedd) in north Wales. It was located at the head of the valley of Cwm Rhwyddfor (or Cwm Rhwyddor) on the pass called Bwlch Llyn Bach between Minffordd and Cross Foxes; Llyn y Tri Greyenyn is the 'llyn bach'' (‘little lake') in the pass's name. Most of the lake has now been infilled in order to improve the road (which formerly skirted the lake). Today, the A487 passes through the site. There is also a small car park on the location of part of the lake (on the northwest side of the road). Some of the remains of the old lake can be seen on the south-east side. As the car park is a convenient place to watch military aircraft on the so-called 'Mach Loop', the car park is sometimes called the ‘Mach Loop' car park.