Maybe try to do it in steps. I didn’t like black coffee when I started drinking it but you get used to it after a while. Try coffee just with milk for a while, then when you get used to that try it black.
It sounds like your body is used to the 3 in ones, have you considered maybe trying to take coffee with normal sugar and milk and then slowly cut the amount of sugar and milk? Or maybe try taking tea instead?
Stop trying to cheat this. Learn to drink black coffee or nothing. There is no short cut. I don’t mean to be harsh but those Nescafé things are literally what you should be avoiding drinking with any regularity if you’re serious about this kind or change.
If this is about the unadulterated flavor and intensity of black coffee, consider brewing a light roast coffee. Light roasts, while subdued in flavor, are enjoyable and certainly preferred to your 3 in 1 using things like maltodextrin and palm oil to lessen the medium dark roast coffee they use in that, product.
From light roasts, you can venture onto medium and even dark roasts. That is, once you develop the appreciation for vivid flavor profiles.
Nescafe sounds like a treat, so use it like a treat. Use it to break your fast, gives you something to look forward to. If that means you drink it at 4, so be it.
Or…Have your Nescafe in the morning, eat a decent breakfast, eat lunch and a snack, and skip dinner.
A lot of people swear by adding a little salt to black coffee, and claim it cuts down on the bitterness by a lot. I haven’t tried it (I happen to like black coffee), but maybe see how that works for you?