As you stated:
This visa is good for a single journey to Malaysia within 3 months from date hereof, provided this passport remains valid.
Malaysian visas are issued for 90 days, and you can be in the country during that time. Once the 90 days are up, you're expected to have left. Any later and you're deemed to have overstayed.
So if you arrive on the last day of your visa, you could be let in, but they'll likely ask when you're leaving, as they'll note it's your last day of being allowed to be in Malaysia. They'll see you're planning on overstaying, and then you'll run into problems.
At this point, you'll need to change travel plans, or apply for a new visa.