Quote Originally Posted by Silver Rusher
Well, it depends what you mean by 'Slav', because many of the peoples who are commonly referred to as Slavs are not ethnically Slavs, just people speaking Slavonic languages. Southern Slavs, for example, are mostly steppe tribes that settled in that area and learned slavonic languages from either people they lived near early on, invaders or the sparse Slavic population of the regions they settled.

Croats are thought to be descended from Alans, but this is only a theory. Otherwise, the origins of the Croat tribe are unknown.
Serbs were a Sarmatian tribe.
Bulgarians were a Turkic people (another branch of the Bulgar tribe was the Volga-Bulgarians, who as you may have known settled along the banks of the Volga and converted to Islam).
Serbs were not a sarmatian tribe. The fact that there was a sarmatian tribe called "serbs" doesn't necesarilly mean that serbs were sarmatians. Toponyms which include the root of the name (srb) exist in a very large area, from middle east and asia to central europe. Sarmatian theory is just one of the many theories of the origin of the name "serbs". Wikipedia:
Earliest historical records of names similar to "Serb"
Here are a few of the earliest quotations from well known ancient geographers and historians:

Herodotus (11,6) (5th century BC), and Diodor from Sicily (1,30) mention the lake named Serbonis (Σερβυνιδοζ) in lower Egypt. However taking the large distances into the account it is highly unlikely that today's Serbs have anything to do with that particular toponym.

Strabo (63 - 19 BC): "the river Kanthos/Skamandros is called Sirbis (Sirbika) by the natives." ( Strabonis rerum geographicarum libri septendicini, Basileza 1571 s. 763).

Tacitus (ca. 50 AD): described the Serboi tribe near the Caucasus, close to the hinterland into the Black Sea. Many consider this theory as a very probable one taking some distant linguistic similarities with today's Caucausus people's such as Ingushi, Chechens etc.

Pliny (69-75 AD): "beside the Cimerians live Meotics, Valians, Serbs (Serboi), Zingians, Psesians." (Historia naturalis, VI, c. 7 & 19 Leipzig 1975). It coincides with the Tacitus's view on Serbian ancient homeland among the Iranian peoples of the Caucausus.

Ptolemy (150 AD): "between the Keraunian mountains and the river Pa, live the Orineians, Valians and Serbs." (Geographia V, s. 9). Ptolemy also mention the city in Pannonia named Serbinum (present day Bosanska Gradiška in Republika Srpska). This well known ancient scientist one more time points out to the Caucasus placing Serbs close to Black Sea riviera.
In the third century Roman emperor Licinius referred to the Carpathians as 'Montes Serrorum' ("Serb mountains").

There is a theory that the name Serbs was a designation for all Slavic peoples in history. The earliest possible association of Serbi with Slavs is from Procopius (6th century), who says that Antae and Sclavenes (Slavs) originally had the common name Sporoi, which has been claimed as a corruption of Srbi (Serbs).

Most non-slavic theories about origin of the croats are from the WWII. Nazi puppet state croatia tried to prove that croatins were not slavs, because slavs weren't "aryan" race. From that time many theories take place, includind scandivian origin of the croats, iranian and so on...

You were right on bulgars, though.