Before you buy anything, you need to understand why. There are three different products that people are interchanging here, and also introducing brand recommendations.
The four different products are:
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External Hard Drive - storage device that directly attaches to a single computer via USB, Firewire, Thunderbolt, etc.
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NAS (Network Attached Storage) - storage device that attaches to a network, usually by hard wire ethernet connection, and recommended Gigabit ethernet.
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Router - a device that connects your internal network to the internet, and provides other functions like assigning IP addresses. Usually also includes a switch to attach hard wired devices and a wireless access point to form a WiFi network.
A NAS or External hard drive can be configured (if the specific device supports it) to provide RAID (redundancy), but may not.
The Time Capsule is a specialized router that includes a single drive NAS within the same box. Since its single drive, there is no redundancy available.
The DROBO is an external hard drive that's claim to fame is that it can support multiple disparate drives and extreme simplicity. Most similar products required similar drives and were more difficult to setup. Then DROBO "bolted on" a network attachment add-on to call it a NAS, but from what I've seen it doesn't compete with a real NAS.
So given your requirement to have multiple computers access your storage, you will want to use a NAS, OR, you will have to keep the computer on all the time that has the external hard drive, AND, turn on sharing. I assume you don't want to do this.
So then your choice is to go with a Time Capsule "all in one" device, or get a router (like the Airport Extreme) and a separate NAS. If you get a separate NAS, it will be more robust and probably provide some redundancy (not to be confused with "backup"). I would not buy a Drobo as a NAS personally. I have a Time Capsule (currently offline), an Airport Extreme, a QNAP TS659 NAS, a Netgear ReadyNAS NV+ NAS, and several other retired routers. You will find similar NAS offerings from Synology, QNAP, and Netgear with a wide range of capability and cost. You need to determine how much storage you need (thus defining how many "bays" you need in a NAS) and also what your purpose will be for the storage.
If you are primarily using the NAS as a Time Machine destination for backup, then you have two copies of what's on it... the original on your computer, and the backup on the NAS... so it may not be necessary to backup the NAS. However, if you are storing data on the NAS that doesn't exist anywhere else, then your NAS should be backed up. This can get expensive if you have a lot of storage. In my case, the reason I have two NAS boxes is so that one backs up the other. RAID provides redundancy to mitigate a single drive failure, but it doesn't help you if the entire NAS or RAID array fails... in which case you'd lose everything. It happens. I'm still exposed because I don't have offsite storage however, but I can't afford cloud backup for the amount of storage I need so this is my best alternative.
My recommendation is that if you can get buy with the Time Capsule, then its the simplest solution you can get. But remember that it isn't backed up, so possibly you could use an inexpensive external hard drive to periodically backup whats on the Time Capsule. If you buy a real NAS box and don't have too many bays, most of them let you attach a USB external hard drive and configure it to be backup for the RAID storage.
If you are new to NAS products, I highly recommend
http://www.eaegis.com/ (I'm not affiliate, just a happy customer). I bought both my NAS boxes from them and there support is outstanding as well as their ability to help you decide what you need. They sell QNAP, Synology, Thecus, and Drobo I believe. They also used to sell ReadyNAS before Netgear bought the line.
