Brahma, Vishnu and Shiva are the Trinity (Trimurthis) responsible for creation, sustenance and annihilation. To complete these jobs each one has a different type of shakti (energy). These shaktis have the name Saraswati, Lakshmi and Parvati respectively. When these 3 unite, it is called Trishakti.
Actually there is only one primordial energy called Adi-shakti or Para-shakti. But based on the task to be completed, this primordial energy transforms into any of the above 3 energies. When the single energy transforms into 3 it is referred to as Trishakti. Remember that prior transformation only the primordial energy existed!
Therefore in the true philosophical sense, Shakti or Trishakti refers to the same energy. However in practical life, it is different. In our life, Trishakti means Goddesses Saraswati, Lakshmi and Parvati while Shakti means Adi para-shakti.