Hello Inzavy, and welcome to ECN!
Not sure what CAD Application you have, so if you have anything other than AutoCAD (or similar applications from the "Autodesk" Company), the following may not work.
Try to perform the following procedures:
[*] Create the 3 new layers (Arch, Elect. Pipe, Equipt.), using the "Layer Management" Tool or Command,
[*] Unlock and/or unfreeze + turn on all existing Layers, also make sure all Blocks and Xrefs are viewable (see note about Blocks and Xrefs, which follows this list),
[*] Select Drawing entities (drawn items) which you plan to have on a certain Layer (only select the ones which will go on the same Layer together),
[*] Invoke the "Properties" command by selecting the "Properties" Icon, which is found on the Object Properties Toolbar,
[*] Click the "Layers" button, then scroll up or down to find the Layer you wish to move the selected items to. Click on that Layer's name, then "OK" the commands as needed to complete the task,
[*] Repeat steps 2 through 5, until all items are moved to the new Layers as needed.
If all entities are moved from the "Unwanted Layers", they may now be "Purged" - via the "Purge/Layers" command.
Before running this command, make sure you have made one of the 3 new layers the "Current" or "Active" Layer - otherwise the Layer which is current at the time of the Purge task will not be removed.
These following Layers may not be removed via "Purge":
***NOTES REGARDING BLOCKS AND XREFS***
- Layer "0"
- Current Layer,
- Layers with Nested Blocks,
- Xref'ed Layers,
- Layers containing certain UCS's, Views & Viewports,
- Layers containing .SHX file entities - like "Shape Fonts".
If a Block has Nested Blocks, or is not a "WBLOCK", moving it to a different Layer may result in part, or all of the Block being lossed.
You may need to Write the Blocks first, which will make them complete entities when they are inserted into the Drawing.
The downside of this is increased File Size.
You may also try to Bind them to the Drawing, or save them as separate files within the same directory.
All Layers on the Externally Referenced entity (the "Xref") will be carried over into the Drawing which the Xref is attached to.
These Layers cannot be Purged while they are attached, so they need to be Purged in the same methods described in steps 1 through 6 above before attaching to the finished Drawing File.
Also, the path & Layer attachment information may need to be edited when the entity is moved to the new Layer, or when the Xref's Layers get Purged - or both - depending on what you have made the default settings for AutoCAD.
The same applies for any OLE items, and Databases.
There are a few Utilities available, which perform these compiling and logistics tasks very well.
Lastly, if needed, you can always compress the Drawing Files with an Archiving type Utility - such as "PKZIP" ("Zip" the drawing files). This technique can reduce file size between 20% and 75% - depending on the complexity of entities, if there are any Raster graphics (entities which are not Vectors), and the Algorithm used by the Utility.