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#1
If I remember correctly, reliability measures interface state changes. 255 is the highest values, i.e. 100% - no state changes.
#2
AD is the measure of trust assigned to a static route or a routing protocol. For example, eBGP route information is normally preferred over IGP route information (for the same prefix).
Metric is the measure of "cost" from router to a prefix within the same dynamic routing protocol. Generally used to select the "better" path, when there's more than one path to the same prefix. For example, RIP uses hop count as its metric.
#3
Well that really depends on the "router" and the "L3 switch". Usually routers will support more media types than L3 switches, which often only support Ethernet. Also routers often will support more "features", e.g. advanced QoS, than a L3 switch. The "true" difference (laugh) is how the vendor labels them. For example, the 6500 is a "L3 switch" and the 7600 is a "router", but their chassis could have the same exact line cards, the same exact supervisor, and (used to) run the same exact IOS. (NB: mind you, the 6500 supported service modules that the 7600 would not, and the 7600 supported "WAN" line cards that the 6500 would not, but these differences were more likely due to marketing rather than hardware technical limitations.)
