Group by 3

var values = JSON.parse(value);
var port;
var result = [];

for (var i = 0; i < values.length; i++) {

port = values[i]['{#SNMPINDEX}'] % 3;
if ( port == 0 ) {port = 3}

result.push({
'{#SNMPINDEX}': values[i]['{#SNMPINDEX}'],
'{#IFALIAS}': values[i]['{#IFALIAS}'],
'{#DEVICE}': 'Device '+(Math.floor((values[i]['{#SNMPINDEX}']-1) / 3)+1),
'{#PORT}': port
});

}

return JSON.stringify(result);