I have worked in all 3 of this company. I can give yiu insight.
Ncs - have money and willing to attract talent (nucleus program) etc but with money come many projects and difficult client and always fighting fire. Cause accepted money and people quitting left and right and they are willing to pay to replace cause fire!!! So be prepared to work. Have learning opportunities as well. People leave after learning for easier life.

Nec - projects that lose out to ncs, come to nec. Usually cause they bid cheaper or ncs can't deliver the past few projects. So customer try new vendor. So cheaper bid. Means cheaper resources. I would say ncs is the power house. Nec is like 3rd class citizen. Not alot of smart people or tech people. Those few also semi exp probably quit from ncs etc. Just people doing C grade work can liao. Likewise bonus sucks. Office also sucks. Ncs office so big. Nec office one building one. Enufff said.

ST - defence project most of the time. No training etc. Cause the people principal engineer, etc all no skills one. Their promotion is based on length of service or how much the senior management like them. So be prepared to take order from clueless people. They want to hire people who have experience but not willing to pay for experience.

Since the pay is the same... It really is...
I would suggest going from ncs, ST to nec.
That way you can get experience from all. Since pay the same.
But start with ncs cause yiu can have more exposure. Training. Etc. Work with more tech savy people.
Once cannot take it. Bring that experience to the lower options.

Don't start with nec or st first.