The sounds of the Tekiah and [the middle sound of] Teruah [or Shevarim] invalidate each other [if one of them is not sounded]. Thus if one knows how to blow the Tekiah but does not know how to blow [neither] the Teruah [or the Shevarim] then he should not blow at all. If however one knows how to blow the Teruah but does not know how to blow the Shevarim then he should blow the set of Teruah three times [Tarat 3x]. Similarly if he knows how to blow the Shevarim but does not know how to blow the Teruah then he should blow the set of Shevarim three times [Tashat 3x]. [However in such a case one does not recite a blessing prior to the blowing. The same applies to one that does not have enough breath to blow more than one set.]
 593/3; Michaber 593/2
 The reason: As there is no Mitzvah involved in simply blowing a Tekiah or Teruah or Shevarim as the Mitzvah is to blow a Tekiah and then a Teruah or Shevarim and then a Tekiah, as explained in 590/1. [Admur ibid]
 The reason: As perhaps the Shevarim or Teruah is the correct middle blow of the Torah and hence he fully fulfills the Mitzvah by blowing it. [Admur ibid; M”A 593/3]
If one knows Tashrat: See Michaber ibid that writes “if one knows how to blow Tashrat but not Tashat and Tarat”; The Mefarshim discuss the obvious question on this wording as certainly if he knows Tashrat he knows the other blows. Various answers are given to this question: 1) The Michaber means to say he does not know how to blow Tashrat in one breath [Taz 593/6] 2) He is unable to blow more than Tashrat. [M”A 593/3] See Nehar Shalom 593; Mamar Mordechai 593/1; Kaf Hachaim 593/6; To note that Admur completely negates this issue and omits the set of Tashrat form this Halacha.
 M”E 593/2; Biur Halacha 593; Kaf Hachaim 593/5
 M”A 593/3