Application.WorksheetFunction.Max(A ,B, C)

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Sub SampleWorksheetFuncMax()

 A=5
 B=20
 C=15

 MsgBox WorksheetFunction.Max(A, B, C)

End Sub
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List of Worksheet Functions Available to Visual Basic.
The following list represents all of the worksheet functions that can be called using the WorkSheetFunction object:

AccrInt, AccrIntM, Acos, Acosh, Aggregate, AmorDegrc, AmorLinc, And, Application, Asc, Asin, Asinh, Atan2, Atanh, AveDev, Average, AverageIf, AverageIfs, BahtText, BesselI, BesselJ, BesselK, BesselY, BetaDist, BetaInv, Bin2Dec, Bin2Hex, Bin2Oct, Binom_Dist, Binom_Inv, BinomDist, Ceiling, Ceiling_Precise, ChiDist, ChiInv, ChiSq_Dist, ChiSq_Dist_RT, ChiSq_Inv, ChiSq_Inv_RT, ChiSq_Test, ChiTest, Choose, Clean, Combin, Complex, Confidence, Confidence_Norm, Confidence_T, Convert, Correl, Cosh, Count, CountA, CountBlank, CountIf, CountIfs, CoupDayBs, CoupDays, CoupDaysNc, CoupNcd, CoupNum, CoupPcd, Covar, Covariance_P, Covariance_S, CritBinom, CumIPmt, CumPrinc, DAverage, Days360, Db, Dbcs, DCount, DCountA, Ddb, Dec2Bin, Dec2Hex, Dec2Oct, Degrees, Delta, DevSq, DGet, Disc, DMax, DMin, Dollar, DollarDe, DollarFr, DProduct, DStDev, DStDevP, DSum, Duration, DVar, DVarP, EDate, Effect, EoMonth, Erf, Erf_Precise, ErfC, Even, Expon_Dist, ExponDist, F_Dist, F_Dist_RT, F_Inv, F_Inv_RT, F_Test, Fact, FactDouble, FDist, Find, FindB, FInv, Fisher, FisherInv, Fixed, Floor, Forecast, Frequency, FTest, Fv, FVSchedule, Gamma_Dist, Gamma_Inv, GammaDist, GammaInv, GammaLn, GammaLn_Precise, Gcd, GeoMean, GeStep, Growth, HarMean, Hex2Bin, Hex2Dec, Hex2Oct, HLookup, HypGeom_Dist, HypGeomDist, IfError, ImAbs, Imaginary, ImArgument, ImConjugate, ImCos, ImDiv, ImExp, ImLn, ImLog10, ImLog2, ImPower, ImProduct, ImReal, ImSin, ImSqrt, ImSub, ImSum, Index, Intercept, IntRate, Ipmt, Irr, IsErr, IsError, IsEven, IsLogical, IsNA, IsNonText, IsNumber, IsOdd, Ispmt, IsText, Kurt, Large, Lcm, LinEst, Ln, Log, Log10, LogEst, LogInv, LogNorm_Dist, LogNorm_Inv, LogNormDist, Lookup, Match, Max, MDeterm, MDuration, Median, Min, MInverse, MIrr, MMult, Mode, Mode_Mult, Mode_Sngl, MRound, MultiNomial, NegBinom_Dist, NegBinomDist, NetworkDays, Nominal, Norm_Dist, Norm_Inv, Norm_S_Dist, Norm_S_Inv, NormDist, NormInv, NormSDist, NormSInv, NPer, Npv, Oct2Bin, Oct2Dec, Oct2Hex, Odd, OddFPrice, OddFYield, OddLPrice, OddLYield, Or, Pearson, Percentile, Percentile_Exc, Percentile_Inc, PercentRank, PercentRank_Exc, PercentRank_Inc, Permut, Phonetic, Pi, Pmt, Poisson, Poisson_Dist, Power, Ppmt, Price, PriceDisc, PriceMat, Prob, Product, Proper, Pv, Quartile, Quartile_Exc, Quartile_Inc, Quotient, Radians, RandBetween, Rank, Rank_Avg, Rank_Eq, Rate, Received, Replace, ReplaceB, Rept, Roman, Round, RoundDown, RoundUp, RSq, RTD, Search, SearchB, SeriesSum, Sinh, Skew, Sln, Slope, Small, SqrtPi, Standardize, StDev, StDev_P, StDev_S, StDevP, StEyx, Substitute, Subtotal, Sum, SumIf, SumIfs, SumProduct, SumSq, SumX2MY2, SumX2PY2, SumXMY2, Syd, T_Dist, T_Dist_2t, T_Dist_RT, T_Inv, T_Inv_2T, T_Test, Tanh, TBillEq, TBillPrice, TBillYield, TDist, Text, TInv, Transpose, Trend, Trim, TrimMean, TTest, USDollar, Var, Var_P, Var_S, VarP, Vdb, VLookup, Weekday, WeekNum, Weibull, Weibull_Dist, WorkDay, WorkDay_Intl, Xirr, Xnpv, YearFrac, YieldDisc, YieldMat, Z_Test, ZTest.