Simple math would indicate you probably want a 100 Mbit fiber connection. Most providers will let you buy a fractional or throttled line that you can scale up or down, so you can get the fiber connection installed, pay for 50 Mbit initially, and scale it up (pay to use more capacity) as needed.
Of course, since you're running a public-facing infrastructure, and presumably can't afford downtime if your single internet connection goes down, you'll want at least a second redundant connection with another provider. Oh, and you'll probably want a second, redundant power feed to the building (two power feeds from two different providers) for the same reason, not to mention a generator. And even then, what if there's a fire (etc.), and the whole building become inaccessible?
Ultimately, I wouldn't feel comfortable hosting public services from an office building (and you guys probably shouldn't, either), so I would advise against taking your webservers out of a datacenter environment. Yes, datacenters are expensive... but how expensive would it be if our website was down for a few days? Quality datacenters have systems in place to ensure high-availability and redundancy everywhere. Office buildings do not. Many of the larger datacenter providers even have their infrastructure setup to allow quick failover or DR to other geographical locations.
You really should be looking at other datacenter providers or options to reduce costs with your current provider, rather than risk bringing your public-facing services into an environment with multiple single points of failure.
I've actually worked somewhere that had a public-facing service hosted out of an office building, and the service and retrofitting required to make the office park redundant for power and electricity (and safeguard against fire and flooding) cost 10's of millions of dollars. For your 3 webservers, I can't imagine that being cheaper than the datacenter option.
