I prefer placing the hose/tube/whatever underneath where it will go. Making sure all three openings are directly under the ports they will connect to. Mark just one which needs cutting and cut it. Hold it underneath again and mark the next and cut it. Then repeat for the final one. Or, if you want/choose, suspend the hose underneath in place using tape, string, or zip-ties applied loosely so the hose just hangs below where it will go. Or just hold it without adjusting its position. Mark all three, remove it and then cut all three. You can leave extra beyond the marks if not feeling confident and trim again.
Check it fits at any time for no sign of kinking.