TransactionDetails
.Join(Transactions, td => td.TransactionId, t => t.Id, (td,t) => new {td,t})
.Join(ProductDetails, ttd => ttd.td.ProductDetailId, pd => pd.Id, (ttd,pd) => new{ttd.td, ttd.t, pd})
.Where(x => x.t.TransactionType == "Sales")
.GroupBy(x => new {x.td.ProductDetailId, x.pd.CostPrice})
.Select(g => new { ProductDetalId = g.Key.ProductDetailId, CostPrice = g.Key.CostPrice, Quantity =  g.Sum(a => a.td.Quantity),Total = g.Count()})
.Select(l => new {Net = (l.CostPrice * l.Quantity)})