Software solution to ensure 100% uptime? No.
You'd need to set up redundant EVERYTHING (hardware and software) to ensure 100% uptime, and honestly even then there really is not a 100% uptime guarantee, it's just getting you as close to that 100% as possible. Even Facebook, Google, Microsoft, etc. have experienced downtime at some point. The more redundant networks and servers you set up will help get you as close to that 100% mark as possible.
If the 3 days you're referring to is very close you really don't have a leg to stand on at this point, but if you have enough time, could take you a day, a week, or a year, depending on the amount of data and complexity of your site that you need to mirror; you can work on setting up a second server at a separate KH location and split the nameserver IPs between the two. To understand this redundant setup have a look at
http://www.wight-hat.com/guides/hosting6.html
This would ensure
better uptime but if the primary system does in fact fail you'd end up having inconsistent data between the two servers and this may cause you more problems than just being down momentarily. But, also remember there's a 99.999% unlikely chance of that happening. To use this arrangement without creating inconsistent data you would need to program your system to mirror data between the two when both servers are back online. This is just one redundancy scheme and if you were to begin setting up your own servers and networks you could have access to many more schemes just like Facebook and Google, but then you're talking huge $$$.
