The single species tank idea is great, but probably beyond the means of most, unless it's a small tank with a fast grower. I've seen anubias-only tanks that were striking enough that I'd like to try that, if the contest were 3 plants, then I would know what 3 Anubias I would want. Only problem, you'd have to check back with me in about 3 years
I say go for it. A 3-species tank would really give some a motivation to do a tank that's more focused than the usual.