Brahma, Vishnu and Shiva are the Trinity (Trimurthis) responsible for creation, sustenance and annihilation of the creation. To complete these jobs each one has a different type of shakti (energy). Their energies (shaktis) are 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.

(Glimpses of Bhaktimala)