set terminal pngcairo size 500, 500 enhanced font 'Verdana, 10'

f(x) = a - b * x

fit f(x) 'data3' u 1:3 via b, a

set xlabel 'T, deg'
set ylabel 'U/F, мН/м'
plot 'data3' using 1:3:2:4 w xyerrorbars title 'uf-plot', f(x) title 'fitting line'

pause -1