In a large, deep, heavy-bottomed pot, combine raspberries, plums and lemon juice. Bring to a boil over high heat, stirring constantly. Add sugar in a steady stream, stirring constantly. Bring to a full boil, stirring constantly to dissolve sugar. Immediately stir in pectin; return to a full boil. Boil hard for 1 minute, stirring constantly. Remove from heat and skim off any foam. Stir for 5 to 8 minutes to prevent floating fruit.
Ladle into sterilized jars to within 1⁄4 inch of rim; wipe rims. Apply prepared lids and rings; tighten rings just until fingertip-tight. Process jars in a boiling water canner for 10 minutes. Transfer jars to a towel-lined surface and let rest at room temperature until set. Check seals; refrigerate any unsealed jars for up to 3 weeks.