The insertion can be done using Gibson assembly. Please follow the link for further detail. It will save lot of time.

https://www.addgene.org/protocols/gibson-assembly/

Now a days, Gene synthesis should be able to construct this full length of insert (i.e., 3 fragments combined) and the synthesized product can be cloned commercially.

With your classical method, its kind of hard. However, you may try this:

a) Make sure that the restriction digestions as mentioned have been completed and properly cleaned of the enzymes. Vector should be adequately digested with both enzymes and dephosphorylated.

b) Set up ligation of the 3 fragments (initial 1:1:1 molar ratio) first; may try the 15 minute or the overnight protocol using 16C or 4-6C (whatever works). Use enough of the 3 fragments so as to monitor on the agarose gel. Check for the proper size ligated product. At this point, this band may be sliced and cleaned.

c) Because of the blunt end cutter, an alternative approach would be setting up A+B fragment ligation and Vector +C ligation separatly.

d) Now set up the final ligation by adding cleaned vector and isolated desired insert (or add the vector to the initial ligation reaction). If there is a unique enzyme site on the polylinker of the vector that is in between SalI and XhoI, but is absent in your insert, then you may set up a restriction digestion of the finally ligated product before transformation.

Good Luck!!