Spending some time with the mach3 manual OR other CNC manuals that explain BASIC CNC and playijng with the machine to learn its operations is going to go a LONG way to helping you understand the process. It has a rather steep learning curve at times.
0,0,0 (;-) WHich one ? Machine coords, user coords, G28, G30 or Fixture offset(g54,g55,g56,etc), G10
?? It all depends on what you need to do.
First thing to do is tell MACH3 where it really is in your 3d space. (Set the machine corrd base). It can be done several ways, Home switches, touching off to a preset spot on the table, manual touch off to the part, probing either the table or part. Basically this allows Mach3 to know where it is compared to your table or part. Once set it remembers where it is in Machine coords reguardless of what you do with offsets.etc. Once you turn OFF mach3 then it is lost and must be reset on startup.
Once done with that NOW you deal in user coords. These area variable as to your needs. TO machine you first establish the Point of origin. THis syncs your Gcode to a specific place on the table and when at this point the DROs will read 0,0,0. Now you can shift this point on the fly with things such as Fixture offsets (G54,G55,G56,etc) This effectively MOVES the point of origin across the table to a new starting point and Mach3 keeps the record of where it moved to in case you need to go back there. You can also use G52/92 offsets to offsets the point on the table.
So you can set up several Fixture offsets for each part on the table and then reset the Point of origin to each part when it time to do the work.
For example I start in G54 and setup the original 0,0,0 and machine the first part. Then switch to G57 which shifts the Poin of Origin to the next part to machine and then machine that part. THis way when you DRAW and Cam the part all the parts can be drawn to have the Point of orgin of the part to say the lower left corner of the part and you do NOT have to know where the part will be machined on the table.
This can also be done with part offsetting with G52/92 or G10.
That is just a nutshell view of the operations . TIme,study and practice will be your friend in this game.
Just a thought, (;-) TP