Isometric dot paper, A3
The isometric lattice as dots on A3, for work that has to be presentable once the construction is done.
Opens with these settings already loaded. Change anything you like, then download as PDF, PNG or SVG.
Dots disappear behind the drawing
A full line grid is easier to construct on, because you can follow an existing line rather than judge alignment between two dots. The cost is that the grid stays visible in the finished sketch and photographs or scans badly. Dots mark the same lattice points and then recede. On A3, where the drawing is likely to be shown to someone or pinned up, that difference matters more than it does on a working A4 sheet.
Count along the axes, not vertically
The 5 mm figure is the distance between adjacent dots measured along the three drawing axes. Rows sit closer together than that, by a factor of about 0.866, which is the sine of 60 degrees. If you are counting dots to keep a solid square, count along an axis. Measuring the vertical gap and assuming it is the spacing is the single most common misreading of isometric paper.
A working pattern worth stealing
Construct on the line grid where following a line is easier, settle the geometry, then redraw the resolved object on dot paper for the version you keep. It sounds like extra work and mostly is not, because the second drawing goes quickly once every decision is already made, and the result is clean enough to hand to somebody.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.