No of keys used in Triple DES Algorithm
The first question is already answered here.

For the second question, a single key $k \in \left\{0,1\right\}^{168}$ can be trivially converted back and forth from three keys $k_1, k_2, k_3 \in \left\{0,1\right\}^{56}$ – there is no semantic difference between the two. DES itself operates only on 56-bit keys, and triple-DES requires three independent DES operations. Software that implements triple-DES can trivially be written to either accept a single 168-bit key or three 56-bit keys.