The scalar product of the vector \[\widehat{i}+\widehat{j}+\widehat{k}\] with a unit vector along the sum of vectors \[2\widehat{i}+4\widehat{j}-5\widehat{k}\] and \[\lambda \widehat{i}+2\widehat{j}+3\widehat{k}\] is equal to one. Find the value of \[\lambda \].