(1) As can be seen from the data in the table, the relationship between the resistance value Rt and the temperature is RT = 20+0.5t.

(2) when t=0℃ and Rt=20,

According to ohm's law, I = ur = ur0+rt = 3v20ω+20ω =? 0.075A，

The voltage U0 = IR0 = 0.075A× 20Ω =1.5v on the constant resistance;

(3) When the temperature is high, the resistance value of the resistor is large, and the resistance value of the fixed-value resistor is too small relative to the resistor, resulting in obvious changes in the voltage representation.

So the answer is: (1) rt = 20+12t;

(2) When the temperature is 0℃, Xiao Ming observes that the reading of laboratory voltmeter is1.5v;

(3) When the temperature is high, the resistance value of the resistor is large, and the resistance value of the fixed-value resistor is too small relative to the resistor, resulting in obvious changes in the voltage representation.