Alan Thicke's Sons Fighting to Protect Estate from His Wife

Alan Thicke's sons -- Robin and Brennan -- believe their dad's third wife is making moves to get more of his estate than she deserves, and they're taking action to nip it in the bud.

The sons, with attorney Alex Weingarten, have filed legal docs in Alan's probate case, claiming his widow, Tanya Callau, is greedy and overreaching to get a bigger piece of the pie than their dad intended in his will. In the docs they say she threatened to go to tabloids if her demands are not met.

They don't mention a specific dollar amount she's allegedly trying to get.

According to the docs ... Alan got rich and famous long before meeting Callau, and they signed a prenup when they wed in 2005. The sons say she's threatening to challenge the prenup.

The sons say Tanya also wants more because she claims to have given up her own career to support Alan and help raise his youngest son, Carter.

According to docs ... the boys say their dad left Tanya 25 percent of his personal effects, 40 percent of his remaining estate, a $500k life insurance policy and said she could live at the ranch.

Robin and Brennan want a judge to enforce the will and the prenup.