I have basic knowledge about these but I want to learn in depth. So what could be the sequence to learn. Also tell where Form, and other elements will come.
First you have to know at least all what's in the ad of this site: https://www.musictheory.net/lessons
to get an idea what sequence you should choose: https://www.orange.k12.nj.us/cms/lib7/NJ01000601/Centricity/Domain/15/MusicTheory_9-12.pdf
you may also just follow the jazz book of mark levine
or any other book about harmony like: https://www.bestmusicteacher.com/download/Tchaikovsky_HarmonyTextbook_Eng.pdf
The basic arrangement of musical sounds horizontally (melody), vertically (harmony), and across time (rhythm) should be studied together. Form and other advanced topics can come once these basic three have been mastered. During the lesson, it is common for the student to demonstrate rhythm practice by tapping the left and right hands on the knees, and melodic practice by performing etudes (musical themes, scales, and arpeggios). The student may then analyze/discuss the harmonic progression of the etude for the teacher. Finally, as the student advances into the significant literature, form, counterpoint, and history may be discussed.
Your question is very broad. Almost too broad to answer, but if you list the key words in your question...
...harmony, melody and rhythm ...form
There is one particular thing that does bring all these elements together in the classical style: the cadence.
But, here is the caveat. I don't mean cadence in the loose sense that many explain it. Many time people list things like
V7|I authentic cadence
IV| plagal cadence without regard for if the progression is the end of a phrase.
Of course the basic definitions of cadence will cover harmony and melody. For example, authentic cadence is harmonically
V|I and melodically something like
^3 ^2 ^1 in the treble and
^5 ^1 in the bass. The rhythmic/metrical aspect can come in through examining the barline.
You also asked about form. This is where a deeper level study of cadence becomes instructive. A cadence isn't a chord progression, it's a form device. This article - Caplin, The Classical Cadence - is a great explanation of that idea.
Studying cadences may seen too basic, but it provides a good integrated view of harmony, melody, rhythm, and form.
Consider things like: harmonically "advanced" concepts like augmented sixth chords are intimately related to the half cadence and melodically are contrapuntal devices as well as examples of tendency tones. Formally the half cadence is essential for understanding the structure of classical phrasing. This is how all the musical elements are integrated in cadences and cadential harmony.
In terms of study tasks you could try playing cadence patterns in all 24 keys, identify cadences and their formal function in real scores staring with small forms like minuets and moving up to large forms like the sonata, and reading a college harmony textbook.