Water commissioners representing Braintree, Holbrook, and Randolph could be nearing a decision about the towns’ future water supply, according to Holbrook Town Administrator William Phelan. The towns have two water treatment facilities, one that serves Braintree and one that serves Holbrook and Randolph, and officials have said both need to be replaced. Proposals have included building a single facility in Braintree that would serve all three communities. The towns need to build a new facility or buy water from the Massachusetts Water Resources Authority, Phelan said. He was not certain of the timeline for a decision.