Isshin sangan means to accept the three dogmas of "Ku (mind that is not bound by anything), Ke (or difference, a thought that things exits temporally) and Chu (or totality)" although the mind of an ordinary person keeps changing all the time.
一心三観とは、凡夫・衆生の心にはつねに一瞬一瞬で変化するが、その中に「空・仮・中」の三諦で観ずることをいう。