如果你用spire.xls （https://www.e-iceblue.cn/Introduce/Free-Spire-XLS-NET.html）, 可以用Chart.PrimaryValueAxis.MajorUnit属性来设置Y轴的单位，小单位也可以通过MinorUnit来设置，不知道epplus有没有对应的属性。

用spire创建line3d的代码如下：

using Spire.Xls;
using Spire.Xls.Charts;

namespace InsertLinChart
{
    class Program
    {
        static void Main(string[] args)
        {
            //创建Workbook
            Workbook wb = new Workbook();
            //获取第一个工作表
            Worksheet sheet = wb.Worksheets[0];
            //添加数据
            sheet.Range["A1"].Value = "Country";
            sheet.Range["A1"].Style.Font.IsBold = true;
            sheet.Range["A2"].Value = "Cuba";
            sheet.Range["A3"].Value = "Mexico";
            sheet.Range["A4"].Value = "France";
            sheet.Range["A5"].Value = "German";
            sheet.Range["B1"].Value = "Sales";
            sheet.Range["B1"].Style.Font.IsBold = true;
            sheet.Range["B2"].NumberValue = 6000;
            sheet.Range["B3"].NumberValue = 8000;
            sheet.Range["B4"].NumberValue = 9000;
            sheet.Range["B5"].NumberValue = 8500;

            //添加Line3D图表
            Chart chart = sheet.Charts.Add();
            chart.ChartType = ExcelChartType.Line3D;

            //设置Y轴单位
            chart.PrimaryValueAxis.MajorUnit = 2000.00;

            //设置数据区域
            chart.DataRange = sheet.Range["A1:B5"];
            chart.SeriesDataFromRange = false;

            //设置图表位置
            chart.LeftColumn = 1;
            chart.TopRow = 5;
            chart.RightColumn = 9;
            chart.BottomRow = 25;

            //设置标题
            chart.ChartTitle = "Market share by country";
            chart.ChartTitleArea.IsBold = true;
            chart.ChartTitleArea.Size = 12;
            foreach (ChartSerie cs in chart.Series)
            {
                cs.DataPoints.DefaultDataPoint.DataLabels.HasPercentage = true;

            }

            chart.Legend.Position = LegendPositionType.Top;
            //保存文档
            wb.SaveToFile("LineChart.xlsx", ExcelVersion.Version2016);
        }
    }
}