PX? That's not an M3+, thats an M6. The LX is the M3+ for Xenon.
I never understood why people like the L. It's energy regen is less than most ships and it can't use Energy Bolt Chainguns (which are amazing for taking out big targets, bit of ammo but no energy consumption). It's not that fast either.
The Sabre is really fast, so most M3s are going to feel very, very slow. If you have about 2 mil for a Scimitar it might be a good ship, but you have to stick with the Terran weapons on it. It's fast and heavily shielded though, better than pretty much anything you can get from the other races. The only limitation really is you have to stick with Terran weapons, which is OK if you like them but I never did like them much.
I gave pretty much the same advice earlier in the thread, but another good ship is the Nova Raider. You can buy it just outside the Terran sectors in Omicron Lyrae (Argon sector) for a bit less than 2.5 mil if I remember correctly. It's a little bit slower and not as heavily shielded, but it can use lots of great weapons. I always put a couple phased repeater guns for most fights and some enerby bolt chainguns with ammo for the real heavy fights.
PRGs are Argon tech so you should be able to find a factory for them in Argon sectors. They are really great guns for most situations. The range is slightly better than the high energy plasma throwers, the shots move extremely fast (so you practically can't miss), and they're even a little bit more efficient for shield damage (especially if you take into account all the missed shots you get on other weapons). Just put as many on as you feel comfortable with - put a bunch if you want short high damage runs where you have to stop for energy, or just a few if you prefer sustained firing and want more room for the other weapons. I usually just put a pair of these and fill the rest with EBCs.
The EBCs are Teladi tech, you should be able to pick some up near Omicron Lyrae (I seem to recall there being some Teladi sectors just west of there with EBC forges and ammo factories). They take ammo, but only weigh 1 each so you should have plenty of cargo room for ammo. You might have to park a freighter on an ammo factory early on to get enough ammo until you can set up your own ammo factory though. These outrange all the other light weapons and the energy use is practically nil. They aren't too bad on the ammo, but you probably want to set up a toggle so you can turn them off and on and only use them on larger ships (PRGs are fine for tearing up most other small craft). These can give you the firepower needed to destroy capital ships in an M3.
You'll need close to 4 mil to get the ship and all the guns (as well as some rep with the Argon and Teladi), but it's totally worth it.