You will find most of the information you're looking for on https://www.neb.com/protocols/2012/12/07/optimizing-restriction-endonuclease-reactions

I routinely digest about 2ug of ~6kb vector in 20ul, but be aware that more than this and you risk over-loading agarose gels, and remember that spin columns have a maximum binding capacity too.

Yes, you can use 3 enzymes at once if your buffer is compatible, but you don't want more than 10% of your reaction volume consisting of enzyme (because of the glycerol).

You will find most of the information you're looking for on https://www.neb.com/protocols/2012/12/07/optimizing-restriction-endonuclease-reactions

I routinely digest about 2ug of ~6kb vector in 20ul, but be aware that more than this and you risk over-loading agarose gels, and remember that spin columns have a maximum binding capacity too.

Yes, you can use 3 enzymes at once if your buffer is compatible, but you don't want more than 10% of your reaction volume consisting of enzyme (because of the glycerol).

Michael is right, you can use 3 enzymes at once, on the other hand we prefer a volume of 50 µl when digesting 2 µg plasmid. To avoid overloading of the gel we use combs with bigger slots (50-100 µl volume).

you can digest the DNA in large quantity by adding the enzymes after two hr of digestion after spinning shortly....