How long are the cables to the sensors ? I have more than 20 DS18B20 with up to 20 meter cables BUT you cannot connect more than one long sensor cable (for each input). The others must be short. If you check the application notes for DS18B20 you will find this limitation. This is because the delays will be too long for some signals.
Try to connect three sensors with short cables. "Short" is certainly less than 1 meter.
AN148: GUIDELINES FOR RELIABLE LONG LINE 1-WIRE NETWORKS