The 3 times distance rule works for me..but what I don’t get is that with the DME not helping here, since it is the direct distance to field only... I somehow need to have the Approx track miles being displayed to compare it with my alt... here again I need a proper path having entered in the FMC before again?
Yes, the approximate track miles is just that.... approximate. Think geometry. What direction are you coming in from? What's the runway alignment?. Distance to the onsite DME is one factor your mental computer, your brain, uses. As you're coming closer, you'll be visual with the field. Now think you're back to flying a C172 around the pattern. The difference is a B737 has better a glide angle than a C172, it's harder to get rid of excess enegy. If you feel you're high on speed and/or altitude - extend the downwind, head for a wider base or whatever is necessary.. If you feel you're good, keep it tight. It's fun! Our manual says wings level at 500 ft.
It'll come to you eventually. You wont be an expert from the get go. The base training will be a good introduction. Depending on airline you're about to join you may get many, or not so many, opportunities to try it out in the future. For the Type Rating you'll have other things to focus on.