Can anyone tell me what is the easiest way to get a 737-400, 757-200 and 747-400 as flight sims on my computer. (or anyone of them)
Buy Microsoft Flight Simulator and then buy the PMDG add-ons for the 737-400 and 747-400. I'm not sure who does the 757; Wilco may have one. I can vouch for the PMDG aircraft, which are legendary in the simulation community.
You need Microsoft Flight Simulator so that you have the basic flight simulation platform. The add-on aircraft (which are actually more expensive than the simulator itself, and for good reason) provide orders of magnitude better realism than the default Boeing airliners that come with the sim. The default aircraft are designed to be easier to fly, so most systems are not simulated at all or greatly simplified. Companies like PMDG specialize in simulating everything that can be simulated, with great attention to detail. Their products are good examples of what this desktop sim can do when it is pushed towards its actual design limits. With each new aircraft that PMDG produces, they do a little better on the details. Their latest effort is the MD-11, which is phenomenal (like the 747-400 that preceded it). They are said to be working on a 777.
There are two versions of MSFS, Flight Simulator 2004, and Flight Simulator X. The latter is the latest version, the former is the version that came just before. Both are still available, with the older version being cheaper. The older version, which is called FS2004 or sometimes FS9, runs a lot faster and has a larger market of add-ons; the most recent version provides more eye candy (no change in basic flight dynamics) and requires a hefty computer in order to get decent performance.