The spec for option type 6 has variable length and can support more than two entries. The length field is 8 bits and represents the number of bytes. 256 / 4 = 64 IP's. Clearly this is well beyond the number that the client must recognize, but specifying 3 entries is likely supported by many clients. It certainly won't hurt anything. orst-case the client will ignore the third.
Whether it makes sense or not is a function of your own infrastructure and availability. A WAN site might have two redundant servers on site and a third remote, for example. There are a number of mechanisms to improve availability of DNS infrastructure, many of which don't require additional IP's to be configured on the client side (i.e. anycast, load balancers).