In like manner in the Sign of the precious Cross, this exclent sacrament of the faith teaches us how to know a mystery. For the joining of three fingers together - the thumb, the ring finger and the little finger, confess the mystery of the three divine hypostases, the Father, the Son, and the Holy Spirit, one God in three persons, while joining together the two other fingers - the foreginger and the middle finger - and extending them signifies the mystery of the Lord Jesus Christ Himself, Who is perfect God and perfect Man. Slightly bending the middle finger confesses the mystery that the Song of God bowed the ehavens nd came down upond the earth and became man for our salvation - Patriarch Joseph of Moscow
Many Old Believers would then use this to argue that the modern sign of the cross is blasphemous because joining the first three forefingers and making the sign of the Cross is heretical since it is placing the Trinity on the Cross, instead of just Christ.
To take this to the Serbian salute, I think you're fine.