Hi, 

Yes, you can achieve this with a CustomConditionalFormatRule. 

Basically, you just need a way of setting the Median, Maxium and Minimum values for the Rule, as you can't do them through a binding. 

Probably the simplest way to do this, is through deriving your own ThreColorScaleConditionalFormatRule

Just follow the steps below: 

1. Derive from ThreeColorScaleConditionalFormatRule:

public class CustomThreeColorScaleConditionalFormatRule : ThreeColorScaleConditionalFormatRule

    {

        protected override IConditionalFormattingRuleProxy CreateProxy()

        {

            return new CustomThreeColorScaleConditionalFormatRuleProxy() { Median = this.MedianValue, Maximum = this.MaximumValue, Minimum = this.MinimumValue };

        }

    }

2. Derive from ThreeColorScaleConditionalFormatRuleProxy (this is the workhorse that performs all of the logic, here, we'll need to set the Median, Maximum, and Minimum values before we execute:

public class CustomThreeColorScaleConditionalFormatRuleProxy : ThreeColorScaleConditionalFormatRuleProxy

    {

        public double? Median

        {

            get { return this.MedianValue; }

            set { this.MedianValue = value; }

        }

        public double? Maximum

        {

            get { return this.MaximumValue; }

            set { this.MaximumValue = value; }

        }

        public double? Minimum

        {

            get { return this.MinimumValue; }

            set { this.MinimumValue = value; }

        }

        protected override Style EvaluateCondition(object sourceDataObject, object sourceDataValue)

        {

            if (sourceDataValue == null)

            {

                return null;

            }

            Summary summary = sourceDataObject as Summary;

            if (summary != null)

            {

                this.MaximumValue = summary.MaxWeight;

                this.MinimumValue = summary.Minweight;

                IQueryable q = summary.Students.AsQueryable();

                SummaryContext sc = SummaryContext.CreateGenericSummary(typeof(Student), "Weight", LinqSummaryOperator.Average);

                this.MedianValue = Convert.ToDouble(sc.Execute(q), CultureInfo.InvariantCulture);

                this.GenerateLineEquations();

            }

            return base.EvaluateCondition(sourceDataObject, sourceDataValue);

        }

    }

3. Use this Rule instead of the in the box ThreColorScaleRule:

 <infragistics:TextColumn Key="OurWeight" Width="*" IsSortable="True" IsSorted ="Ascending">

                    <infragistics:TextColumn.ConditionalFormatCollection>

                        <local:CustomThreeColorScaleConditionalFormatRule  MaximumColor="Salmon" MedianColor="#FFD6C347" MinimumColor="#FF76D264" MaximumValue="{Binding RowData.MaxWeight}" MinimumValue="{Binding RowData.Minweight}"/>

                    </infragistics:TextColumn.ConditionalFormatCollection>

Hope this helps, 

-SteveZ