When you say "find the trendline of each curve" I assume you mean that you have several sets of (x,y) data and you're doing a (linear?) fit to each of those sets? If so, forget Excel and just do the whole lot in MATLAB!
% Make some pretend data
% Each row is a data set
x = (0:5)';
y = [3 + 2*x,3.2 + 1.6*x,2.6 + 2.2*x] + 0.2*randn(6,3);
% Make design matrix
M = [x,x.^0];
% Solve for coefficients
C = M\y;
% Evaluate fits
yfit = M*C;
% Take average of fits
avgyfit = mean(yfit,2);
% See the results
plot(x,y,'x')
hold on
plot(x,yfit)
plot(x,avgyfit,'k','linewidth',2)
