data = Import["/Users/webers/Documents/Teaching/MethodenkursMathematicaMatlabPhysChem/SS2016/MathematicaPart/SampleData/sampledata01.xlsx", {"Data", 1}]

plotdata = ListPlot[data, AxesLabel ->{"x", "y"}]

fitdata = Fit[data, {1/(1 + (y+1)^2), 1/(1 + (y-1)^2), 1/(1 + (y-3)^2)},y]

plotfit = Plot[fitdata, {y,-5,5}, AxesLabel ->{"x", "(Subscript[f, a])(x)"}]

Show[plotfit, plotdata, AxesLabel ->{"x", "(Subscript[f, a])(x)"}, PlotRange-> All]

nlm = NonlinearModelFit[data, a/(b + (x+1)^2) + c/(d + (x-1)^2) + e/(f + (x-3)^2), {a, b, c, d, e, f}, x]

Show[ListPlot[data], Plot[nlm[x], {x, -5, 5}],AxesLabel ->{"x", "(Subscript[f, b])(x)"}, PlotRange-> All]