Garudapurana 1.32.24

1.32.24

maṇḍalatrayamadhye tu kīrtitā hyasanasthitiḥ /
pūrvādipadmapatreṣu pūjyāḥ saṃkarṣaṇādayaḥ //

The position of the seat is taught in the middle of the three circles;