> rov:=diff(x(t),t,t,t)+3=0;

rov := (diff(x(t), `$`(t, 3)))+3 = 0

> poc1:=x(0)=5;

poc1 := x(0) = 5

> poc2:=D(x)(0)=2;

poc2 := D(x)(0) = 2

> dsolve({rov,poc1,poc2,D(D(x))(0)=3},x(t));

x(t) = -1/2*t^3+3/2*t^2+2*t+5

>